diff --git a/pkg/core/blockchain.go b/pkg/core/blockchain.go
index d9cc75234..909b03106 100644
--- a/pkg/core/blockchain.go
+++ b/pkg/core/blockchain.go
@@ -694,7 +694,7 @@ func (bc *Blockchain) handleNotification(note *state.NotificationEvent, d *dao.C
 		}
 		amount = bigint.FromBytes(bs)
 	}
-	bc.processNEP5Transfer(d, h, b, note.ScriptHash, from, to, amount.Int64())
+	bc.processNEP5Transfer(d, h, b, note.ScriptHash, from, to, amount)
 }
 
 func parseUint160(addr []byte) util.Uint160 {
@@ -704,7 +704,7 @@ func parseUint160(addr []byte) util.Uint160 {
 	return util.Uint160{}
 }
 
-func (bc *Blockchain) processNEP5Transfer(cache *dao.Cached, h util.Uint256, b *block.Block, sc util.Uint160, from, to []byte, amount int64) {
+func (bc *Blockchain) processNEP5Transfer(cache *dao.Cached, h util.Uint256, b *block.Block, sc util.Uint160, from, to []byte, amount *big.Int) {
 	toAddr := parseUint160(to)
 	fromAddr := parseUint160(from)
 	transfer := &state.NEP5Transfer{
@@ -721,11 +721,10 @@ func (bc *Blockchain) processNEP5Transfer(cache *dao.Cached, h util.Uint256, b *
 			return
 		}
 		bs := balances.Trackers[sc]
-		bs.Balance -= amount
+		bs.Balance = *new(big.Int).Sub(&bs.Balance, amount)
 		bs.LastUpdatedBlock = b.Index
 		balances.Trackers[sc] = bs
-
-		transfer.Amount = -amount
+		transfer.Amount = *new(big.Int).Sub(&transfer.Amount, amount)
 		isBig, err := cache.AppendNEP5Transfer(fromAddr, balances.NextTransferBatch, transfer)
 		if err != nil {
 			return
@@ -743,11 +742,11 @@ func (bc *Blockchain) processNEP5Transfer(cache *dao.Cached, h util.Uint256, b *
 			return
 		}
 		bs := balances.Trackers[sc]
-		bs.Balance += amount
+		bs.Balance = *new(big.Int).Add(&bs.Balance, amount)
 		bs.LastUpdatedBlock = b.Index
 		balances.Trackers[sc] = bs
 
-		transfer.Amount = amount
+		transfer.Amount = *amount
 		isBig, err := cache.AppendNEP5Transfer(toAddr, balances.NextTransferBatch, transfer)
 		if err != nil {
 			return
@@ -788,23 +787,24 @@ func (bc *Blockchain) GetNEP5Balances(acc util.Uint160) *state.NEP5Balances {
 }
 
 // GetUtilityTokenBalance returns utility token (GAS) balance for the acc.
-func (bc *Blockchain) GetUtilityTokenBalance(acc util.Uint160) int64 {
+func (bc *Blockchain) GetUtilityTokenBalance(acc util.Uint160) *big.Int {
 	bs, err := bc.dao.GetNEP5Balances(acc)
 	if err != nil {
-		return 0
+		return big.NewInt(0)
 	}
-	return bs.Trackers[bc.contracts.GAS.Hash].Balance
+	balance := bs.Trackers[bc.contracts.GAS.Hash].Balance
+	return &balance
 }
 
 // GetGoverningTokenBalance returns governing token (NEO) balance and the height
 // of the last balance change for the account.
-func (bc *Blockchain) GetGoverningTokenBalance(acc util.Uint160) (int64, uint32) {
+func (bc *Blockchain) GetGoverningTokenBalance(acc util.Uint160) (*big.Int, uint32) {
 	bs, err := bc.dao.GetNEP5Balances(acc)
 	if err != nil {
-		return 0, 0
+		return big.NewInt(0), 0
 	}
 	neo := bs.Trackers[bc.contracts.NEO.Hash]
-	return neo.Balance, neo.LastUpdatedBlock
+	return &neo.Balance, neo.LastUpdatedBlock
 }
 
 // LastBatch returns last persisted storage batch.
@@ -1062,7 +1062,7 @@ func (bc *Blockchain) UnsubscribeFromExecutions(ch chan<- *state.AppExecResult)
 // amount of NEO between specified blocks. The amount of NEO being passed is in
 // its natural non-divisible form (1 NEO as 1, 2 NEO as 2, no multiplication by
 // 10⁸ is needed as for Fixed8).
-func (bc *Blockchain) CalculateClaimable(value int64, startHeight, endHeight uint32) int64 {
+func (bc *Blockchain) CalculateClaimable(value *big.Int, startHeight, endHeight uint32) *big.Int {
 	var amount int64
 	di := uint32(bc.decrementInterval)
 
@@ -1088,7 +1088,7 @@ func (bc *Blockchain) CalculateClaimable(value int64, startHeight, endHeight uin
 		amount += int64(iend-istart) * int64(bc.generationAmount[ustart])
 	}
 
-	return amount * value
+	return new(big.Int).Mul(big.NewInt(amount), value)
 }
 
 // FeePerByte returns transaction network fee per byte.
@@ -1148,7 +1148,7 @@ func (bc *Blockchain) verifyTx(t *transaction.Transaction, block *block.Block) e
 	}
 	balance := bc.GetUtilityTokenBalance(t.Sender)
 	need := t.SystemFee + t.NetworkFee
-	if balance < need {
+	if balance.Cmp(big.NewInt(need)) < 0 {
 		return errors.Errorf("insufficient funds: balance is %v, need: %v", balance, need)
 	}
 	size := io.GetVarSize(t)
diff --git a/pkg/core/blockchain_test.go b/pkg/core/blockchain_test.go
index f2269dff5..adbe56f2f 100644
--- a/pkg/core/blockchain_test.go
+++ b/pkg/core/blockchain_test.go
@@ -1,6 +1,7 @@
 package core
 
 import (
+	"math/big"
 	"testing"
 	"time"
 
@@ -196,23 +197,23 @@ func TestGetClaimable(t *testing.T) {
 	require.NoError(t, err)
 
 	t.Run("first generation period", func(t *testing.T) {
-		amount := bc.CalculateClaimable(1, 0, 2)
-		require.EqualValues(t, 8, amount)
+		amount := bc.CalculateClaimable(big.NewInt(1), 0, 2)
+		require.EqualValues(t, big.NewInt(8), amount)
 	})
 
 	t.Run("a number of full periods", func(t *testing.T) {
-		amount := bc.CalculateClaimable(1, 0, 6)
-		require.EqualValues(t, 4+4+3+3+2+2, amount)
+		amount := bc.CalculateClaimable(big.NewInt(1), 0, 6)
+		require.EqualValues(t, big.NewInt(4+4+3+3+2+2), amount)
 	})
 
 	t.Run("start from the 2-nd block", func(t *testing.T) {
-		amount := bc.CalculateClaimable(1, 1, 7)
-		require.EqualValues(t, 4+3+3+2+2+1, amount)
+		amount := bc.CalculateClaimable(big.NewInt(1), 1, 7)
+		require.EqualValues(t, big.NewInt(4+3+3+2+2+1), amount)
 	})
 
 	t.Run("end height after generation has ended", func(t *testing.T) {
-		amount := bc.CalculateClaimable(1, 1, 10)
-		require.EqualValues(t, 4+3+3+2+2+1+1, amount)
+		amount := bc.CalculateClaimable(big.NewInt(1), 1, 10)
+		require.EqualValues(t, big.NewInt(4+3+3+2+2+1+1), amount)
 	})
 }
 
diff --git a/pkg/core/blockchainer/blockchainer.go b/pkg/core/blockchainer/blockchainer.go
index dae93659d..ce723614b 100644
--- a/pkg/core/blockchainer/blockchainer.go
+++ b/pkg/core/blockchainer/blockchainer.go
@@ -1,6 +1,8 @@
 package blockchainer
 
 import (
+	"math/big"
+
 	"github.com/nspcc-dev/neo-go/pkg/config"
 	"github.com/nspcc-dev/neo-go/pkg/core/block"
 	"github.com/nspcc-dev/neo-go/pkg/core/mempool"
@@ -19,13 +21,13 @@ type Blockchainer interface {
 	AddHeaders(...*block.Header) error
 	AddBlock(*block.Block) error
 	BlockHeight() uint32
-	CalculateClaimable(value int64, startHeight, endHeight uint32) int64
+	CalculateClaimable(value *big.Int, startHeight, endHeight uint32) *big.Int
 	Close()
 	HeaderHeight() uint32
 	GetBlock(hash util.Uint256) (*block.Block, error)
 	GetContractState(hash util.Uint160) *state.Contract
 	GetEnrollments() ([]state.Validator, error)
-	GetGoverningTokenBalance(acc util.Uint160) (int64, uint32)
+	GetGoverningTokenBalance(acc util.Uint160) (*big.Int, uint32)
 	GetHeaderHash(int) util.Uint256
 	GetHeader(hash util.Uint256) (*block.Header, error)
 	CurrentHeaderHash() util.Uint256
diff --git a/pkg/core/helper_test.go b/pkg/core/helper_test.go
index ec59d52c6..32ae216de 100644
--- a/pkg/core/helper_test.go
+++ b/pkg/core/helper_test.go
@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
+	"math/big"
 	"os"
 	"testing"
 	"time"
@@ -181,7 +182,7 @@ func TestCreateBasicChain(t *testing.T) {
 	priv0 := testchain.PrivateKeyByID(0)
 	priv0ScriptHash := priv0.GetScriptHash()
 
-	require.Equal(t, int64(0), bc.GetUtilityTokenBalance(priv0ScriptHash))
+	require.Equal(t, big.NewInt(0), bc.GetUtilityTokenBalance(priv0ScriptHash))
 	// Move some NEO to one simple account.
 	txMoveNeo := newNEP5Transfer(neoHash, neoOwner, priv0ScriptHash, neoAmount)
 	txMoveNeo.ValidUntilBlock = validUntilBlock
@@ -211,7 +212,7 @@ func TestCreateBasicChain(t *testing.T) {
 	t.Logf("txMoveNeo: %s", txMoveNeo.Hash().StringLE())
 	t.Logf("txMoveGas: %s", txMoveGas.Hash().StringLE())
 
-	require.True(t, bc.GetUtilityTokenBalance(priv0ScriptHash) >= 1000*native.GASFactor)
+	require.True(t, bc.GetUtilityTokenBalance(priv0ScriptHash).Cmp(big.NewInt(1000*native.GASFactor)) >= 0)
 	// info for getblockheader rpc tests
 	t.Logf("header hash: %s", b.Hash().StringLE())
 	buf := io.NewBufBinWriter()
diff --git a/pkg/core/mempool/feer.go b/pkg/core/mempool/feer.go
index 9a22295f6..b2333f755 100644
--- a/pkg/core/mempool/feer.go
+++ b/pkg/core/mempool/feer.go
@@ -1,11 +1,13 @@
 package mempool
 
 import (
+	"math/big"
+
 	"github.com/nspcc-dev/neo-go/pkg/util"
 )
 
 // Feer is an interface that abstract the implementation of the fee calculation.
 type Feer interface {
 	FeePerByte() int64
-	GetUtilityTokenBalance(util.Uint160) int64
+	GetUtilityTokenBalance(util.Uint160) *big.Int
 }
diff --git a/pkg/core/mempool/mem_pool.go b/pkg/core/mempool/mem_pool.go
index b20318c4f..72703b9c4 100644
--- a/pkg/core/mempool/mem_pool.go
+++ b/pkg/core/mempool/mem_pool.go
@@ -2,6 +2,7 @@ package mempool
 
 import (
 	"errors"
+	"math/big"
 	"sort"
 	"sync"
 	"time"
@@ -35,7 +36,7 @@ type items []*item
 // utilityBalanceAndFees stores sender's balance and overall fees of
 // sender's transactions which are currently in mempool
 type utilityBalanceAndFees struct {
-	balance int64
+	balance *big.Int
 	feeSum  int64
 }
 
@@ -124,7 +125,7 @@ func (mp *Pool) checkBalanceAndUpdate(tx *transaction.Transaction, feer Feer) bo
 		mp.fees[tx.Sender] = senderFee
 	}
 	needFee := senderFee.feeSum + tx.SystemFee + tx.NetworkFee
-	if senderFee.balance < needFee {
+	if senderFee.balance.Cmp(big.NewInt(needFee)) < 0 {
 		return false
 	}
 	return true
diff --git a/pkg/core/mempool/mem_pool_test.go b/pkg/core/mempool/mem_pool_test.go
index c7b8896cb..c3c5d5a15 100644
--- a/pkg/core/mempool/mem_pool_test.go
+++ b/pkg/core/mempool/mem_pool_test.go
@@ -1,6 +1,7 @@
 package mempool
 
 import (
+	"math/big"
 	"sort"
 	"testing"
 
@@ -16,13 +17,13 @@ type FeerStub struct {
 	feePerByte int64
 }
 
-const balance = 10000000
+var balance = big.NewInt(10000000)
 
 func (fs *FeerStub) FeePerByte() int64 {
 	return fs.feePerByte
 }
 
-func (fs *FeerStub) GetUtilityTokenBalance(uint160 util.Uint160) int64 {
+func (fs *FeerStub) GetUtilityTokenBalance(uint160 util.Uint160) *big.Int {
 	return balance
 }
 
@@ -184,7 +185,7 @@ func TestMemPoolFees(t *testing.T) {
 	mp := NewMemPool(10)
 	sender0 := util.Uint160{1, 2, 3}
 	tx0 := transaction.New(netmode.UnitTestNet, []byte{byte(opcode.PUSH1)}, 0)
-	tx0.NetworkFee = balance + 1
+	tx0.NetworkFee = balance.Int64() + 1
 	tx0.Sender = sender0
 	// insufficient funds to add transaction, but balance should be stored
 	require.Equal(t, false, mp.Verify(tx0, &FeerStub{}))
@@ -195,9 +196,10 @@ func TestMemPoolFees(t *testing.T) {
 		feeSum:  0,
 	}, mp.fees[sender0])
 
+	balancePart := new(big.Int).Div(balance, big.NewInt(4))
 	// no problems with adding another transaction with lower fee
 	tx1 := transaction.New(netmode.UnitTestNet, []byte{byte(opcode.PUSH1)}, 0)
-	tx1.NetworkFee = balance * 0.7
+	tx1.NetworkFee = balancePart.Int64()
 	tx1.Sender = sender0
 	require.NoError(t, mp.Add(tx1, &FeerStub{}))
 	require.Equal(t, 1, len(mp.fees))
@@ -208,14 +210,14 @@ func TestMemPoolFees(t *testing.T) {
 
 	// balance shouldn't change after adding one more transaction
 	tx2 := transaction.New(netmode.UnitTestNet, []byte{byte(opcode.PUSH1)}, 0)
-	tx2.NetworkFee = balance * 0.3
+	tx2.NetworkFee = new(big.Int).Sub(balance, balancePart).Int64()
 	tx2.Sender = sender0
 	require.NoError(t, mp.Add(tx2, &FeerStub{}))
 	require.Equal(t, 2, len(mp.verifiedTxes))
 	require.Equal(t, 1, len(mp.fees))
 	require.Equal(t, utilityBalanceAndFees{
 		balance: balance,
-		feeSum:  balance,
+		feeSum:  balance.Int64(),
 	}, mp.fees[sender0])
 
 	// can't add more transactions as we don't have enough GAS
@@ -227,7 +229,7 @@ func TestMemPoolFees(t *testing.T) {
 	require.Equal(t, 1, len(mp.fees))
 	require.Equal(t, utilityBalanceAndFees{
 		balance: balance,
-		feeSum:  balance,
+		feeSum:  balance.Int64(),
 	}, mp.fees[sender0])
 
 	// check whether sender's fee updates correctly
diff --git a/pkg/core/native/native_neo.go b/pkg/core/native/native_neo.go
index 0d3addd68..6d169eaa4 100644
--- a/pkg/core/native/native_neo.go
+++ b/pkg/core/native/native_neo.go
@@ -197,9 +197,9 @@ func (n *NEO) distributeGas(ic *interop.Context, h util.Uint160, acc *state.NEOB
 	if ic.Block == nil || ic.Block.Index == 0 {
 		return nil
 	}
-	gen := ic.Chain.CalculateClaimable(acc.Balance.Int64(), acc.BalanceHeight, ic.Block.Index)
+	gen := ic.Chain.CalculateClaimable(&acc.Balance, acc.BalanceHeight, ic.Block.Index)
 	acc.BalanceHeight = ic.Block.Index
-	n.GAS.mint(ic, h, big.NewInt(gen))
+	n.GAS.mint(ic, h, gen)
 	return nil
 }
 
@@ -212,8 +212,8 @@ func (n *NEO) unclaimedGas(ic *interop.Context, args []stackitem.Item) stackitem
 	}
 	tr := bs.Trackers[n.Hash]
 
-	gen := ic.Chain.CalculateClaimable(tr.Balance, tr.LastUpdatedBlock, end)
-	return stackitem.NewBigInteger(big.NewInt(gen))
+	gen := ic.Chain.CalculateClaimable(&tr.Balance, tr.LastUpdatedBlock, end)
+	return stackitem.NewBigInteger(gen)
 }
 
 func (n *NEO) registerValidator(ic *interop.Context, args []stackitem.Item) stackitem.Item {
diff --git a/pkg/core/native/native_nep5.go b/pkg/core/native/native_nep5.go
index 5fa0a74b3..40ee7cbac 100644
--- a/pkg/core/native/native_nep5.go
+++ b/pkg/core/native/native_nep5.go
@@ -214,7 +214,7 @@ func (c *nep5TokenNative) balanceOf(ic *interop.Context, args []stackitem.Item)
 		panic(err)
 	}
 	balance := bs.Trackers[c.Hash].Balance
-	return stackitem.NewBigInteger(big.NewInt(balance))
+	return stackitem.NewBigInteger(&balance)
 }
 
 func (c *nep5TokenNative) mint(ic *interop.Context, h util.Uint160, amount *big.Int) {
diff --git a/pkg/core/state/nep5.go b/pkg/core/state/nep5.go
index ee552af7e..ad89ffac9 100644
--- a/pkg/core/state/nep5.go
+++ b/pkg/core/state/nep5.go
@@ -1,6 +1,9 @@
 package state
 
 import (
+	"math/big"
+
+	"github.com/nspcc-dev/neo-go/pkg/encoding/bigint"
 	"github.com/nspcc-dev/neo-go/pkg/io"
 	"github.com/nspcc-dev/neo-go/pkg/util"
 )
@@ -8,7 +11,7 @@ import (
 // NEP5Tracker contains info about a single account in a NEP5 contract.
 type NEP5Tracker struct {
 	// Balance is the current balance of the account.
-	Balance int64
+	Balance big.Int
 	// LastUpdatedBlock is a number of block when last `transfer` to or from the
 	// account occured.
 	LastUpdatedBlock uint32
@@ -31,7 +34,7 @@ type NEP5Transfer struct {
 	To util.Uint160
 	// Amount is the amount of tokens transferred.
 	// It is negative when tokens are sent and positive if they are received.
-	Amount int64
+	Amount big.Int
 	// Block is a number of block when the event occured.
 	Block uint32
 	// Timestamp is the timestamp of the block where transfer occured.
@@ -118,13 +121,13 @@ func (lg *NEP5TransferLog) Size() int {
 
 // EncodeBinary implements io.Serializable interface.
 func (t *NEP5Tracker) EncodeBinary(w *io.BinWriter) {
-	w.WriteU64LE(uint64(t.Balance))
+	w.WriteVarBytes(bigint.ToBytes(&t.Balance))
 	w.WriteU32LE(t.LastUpdatedBlock)
 }
 
 // DecodeBinary implements io.Serializable interface.
 func (t *NEP5Tracker) DecodeBinary(r *io.BinReader) {
-	t.Balance = int64(r.ReadU64LE())
+	t.Balance = *bigint.FromBytes(r.ReadVarBytes())
 	t.LastUpdatedBlock = r.ReadU32LE()
 }
 
@@ -136,7 +139,9 @@ func (t *NEP5Transfer) EncodeBinary(w *io.BinWriter) {
 	w.WriteBytes(t.To[:])
 	w.WriteU32LE(t.Block)
 	w.WriteU64LE(t.Timestamp)
-	w.WriteU64LE(uint64(t.Amount))
+	amountBytes := bigint.ToBytes(&t.Amount)
+	w.WriteU64LE(uint64(len(amountBytes)))
+	w.WriteBytes(amountBytes)
 }
 
 // DecodeBinary implements io.Serializable interface.
@@ -152,6 +157,9 @@ func (t *NEP5Transfer) DecodeBinaryReturnCount(r *io.BinReader) int {
 	r.ReadBytes(t.To[:])
 	t.Block = r.ReadU32LE()
 	t.Timestamp = r.ReadU64LE()
-	t.Amount = int64(r.ReadU64LE())
-	return util.Uint160Size*3 + 8 + 4 + 8 + util.Uint256Size
+	amountLen := r.ReadU64LE()
+	amountBytes := make([]byte, amountLen)
+	r.ReadBytes(amountBytes)
+	t.Amount = *bigint.FromBytes(amountBytes)
+	return util.Uint160Size*3 + 8 + 4 + (8 + len(amountBytes)) + +util.Uint256Size
 }
diff --git a/pkg/core/state/nep5_test.go b/pkg/core/state/nep5_test.go
index d95dbc3ff..9681fe2ec 100644
--- a/pkg/core/state/nep5_test.go
+++ b/pkg/core/state/nep5_test.go
@@ -1,6 +1,7 @@
 package state
 
 import (
+	"math/big"
 	"math/rand"
 	"testing"
 	"time"
@@ -40,7 +41,7 @@ func TestNEP5TransferLog_Append(t *testing.T) {
 
 func TestNEP5Tracker_EncodeBinary(t *testing.T) {
 	expected := &NEP5Tracker{
-		Balance:          int64(rand.Uint64()),
+		Balance:          *big.NewInt(int64(rand.Uint64())),
 		LastUpdatedBlock: rand.Uint32(),
 	}
 
@@ -52,7 +53,7 @@ func TestNEP5Transfer_DecodeBinary(t *testing.T) {
 		Asset:     util.Uint160{1, 2, 3},
 		From:      util.Uint160{5, 6, 7},
 		To:        util.Uint160{8, 9, 10},
-		Amount:    42,
+		Amount:    *big.NewInt(42),
 		Block:     12345,
 		Timestamp: 54321,
 		Tx:        util.Uint256{8, 5, 3},
@@ -75,7 +76,7 @@ func TestNEP5TransferSize(t *testing.T) {
 
 func randomTransfer(r *rand.Rand) *NEP5Transfer {
 	return &NEP5Transfer{
-		Amount: int64(r.Uint64()),
+		Amount: *big.NewInt(int64(r.Uint64())),
 		Block:  r.Uint32(),
 		Asset:  random.Uint160(),
 		From:   random.Uint160(),
diff --git a/pkg/network/helper_test.go b/pkg/network/helper_test.go
index f6f66dcdb..cc7dbcb7c 100644
--- a/pkg/network/helper_test.go
+++ b/pkg/network/helper_test.go
@@ -1,6 +1,7 @@
 package network
 
 import (
+	"math/big"
 	"math/rand"
 	"net"
 	"strconv"
@@ -31,7 +32,7 @@ func (chain testChain) ApplyPolicyToTxSet([]*transaction.Transaction) []*transac
 func (chain testChain) GetConfig() config.ProtocolConfiguration {
 	panic("TODO")
 }
-func (chain testChain) CalculateClaimable(int64, uint32, uint32) int64 {
+func (chain testChain) CalculateClaimable(*big.Int, uint32, uint32) *big.Int {
 	panic("TODO")
 }
 
@@ -123,11 +124,11 @@ func (chain testChain) GetMemPool() *mempool.Pool {
 	panic("TODO")
 }
 
-func (chain testChain) GetGoverningTokenBalance(acc util.Uint160) (int64, uint32) {
+func (chain testChain) GetGoverningTokenBalance(acc util.Uint160) (*big.Int, uint32) {
 	panic("TODO")
 }
 
-func (chain testChain) GetUtilityTokenBalance(uint160 util.Uint160) int64 {
+func (chain testChain) GetUtilityTokenBalance(uint160 util.Uint160) *big.Int {
 	panic("TODO")
 }
 
diff --git a/pkg/rpc/server/server.go b/pkg/rpc/server/server.go
index edea05edf..a597bfe7e 100644
--- a/pkg/rpc/server/server.go
+++ b/pkg/rpc/server/server.go
@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"math"
+	"math/big"
 	"net"
 	"net/http"
 	"strconv"
@@ -512,7 +513,7 @@ func (s *Server) getNEP5Balances(ps request.Params) (interface{}, *response.Erro
 			if err != nil {
 				continue
 			}
-			amount := amountToString(bal.Balance, dec)
+			amount := amountToString(&bal.Balance, dec)
 			bs.Balances = append(bs.Balances, result.NEP5Balance{
 				Asset:       h,
 				Amount:      amount,
@@ -547,8 +548,8 @@ func (s *Server) getNEP5Transfers(ps request.Params) (interface{}, *response.Err
 		if err != nil {
 			return nil
 		}
-		if tr.Amount > 0 { // token was received
-			transfer.Amount = amountToString(tr.Amount, d)
+		if tr.Amount.Sign() > 0 { // token was received
+			transfer.Amount = amountToString(&tr.Amount, d)
 			if !tr.From.Equals(util.Uint160{}) {
 				transfer.Address = address.Uint160ToString(tr.From)
 			}
@@ -556,7 +557,7 @@ func (s *Server) getNEP5Transfers(ps request.Params) (interface{}, *response.Err
 			return nil
 		}
 
-		transfer.Amount = amountToString(-tr.Amount, d)
+		transfer.Amount = amountToString(new(big.Int).Neg(&tr.Amount), d)
 		if !tr.To.Equals(util.Uint160{}) {
 			transfer.Address = address.Uint160ToString(tr.To)
 		}
@@ -569,15 +570,14 @@ func (s *Server) getNEP5Transfers(ps request.Params) (interface{}, *response.Err
 	return bs, nil
 }
 
-func amountToString(amount int64, decimals int64) string {
+func amountToString(amount *big.Int, decimals int64) string {
 	if decimals == 0 {
-		return strconv.FormatInt(amount, 10)
+		return amount.String()
 	}
 	pow := int64(math.Pow10(int(decimals)))
-	q := amount / pow
-	r := amount % pow
-	if r == 0 {
-		return strconv.FormatInt(q, 10)
+	q, r := new(big.Int).DivMod(amount, big.NewInt(pow), new(big.Int))
+	if r.Sign() == 0 {
+		return q.String()
 	}
 	fs := fmt.Sprintf("%%d.%%0%dd", decimals)
 	return fmt.Sprintf(fs, q, r)
@@ -783,11 +783,11 @@ func (s *Server) getUnclaimedGas(ps request.Params) (interface{}, *response.Erro
 	}
 
 	neo, neoHeight := s.chain.GetGoverningTokenBalance(u)
-	if neo == 0 {
+	if neo.Sign() == 0 {
 		return "0", nil
 	}
 	gas := s.chain.CalculateClaimable(neo, neoHeight, s.chain.BlockHeight()+1) // +1 as in C#, for the next block.
-	return strconv.FormatInt(gas, 10), nil
+	return gas.String(), nil
 }
 
 // getValidators returns the current NEO consensus nodes information and voting status.
diff --git a/pkg/rpc/server/server_helper_test.go b/pkg/rpc/server/server_helper_test.go
index c36333f9d..391fdf7da 100644
--- a/pkg/rpc/server/server_helper_test.go
+++ b/pkg/rpc/server/server_helper_test.go
@@ -1,6 +1,7 @@
 package server
 
 import (
+	"math/big"
 	"net/http"
 	"net/http/httptest"
 	"os"
@@ -90,6 +91,6 @@ func (fs *FeerStub) FeePerByte() int64 {
 	return 0
 }
 
-func (fs *FeerStub) GetUtilityTokenBalance(acc util.Uint160) int64 {
-	return 1000000 * native.GASFactor
+func (fs *FeerStub) GetUtilityTokenBalance(acc util.Uint160) *big.Int {
+	return big.NewInt(1000000 * native.GASFactor)
 }
diff --git a/pkg/rpc/server/server_test.go b/pkg/rpc/server/server_test.go
index 8ca736379..59100f6c3 100644
--- a/pkg/rpc/server/server_test.go
+++ b/pkg/rpc/server/server_test.go
@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
+	"math/big"
 	"net/http"
 	"net/http/httptest"
 	"reflect"
@@ -270,7 +271,7 @@ var rpcTestCases = map[string][]rpcTestCase{
 								Timestamp: b.Timestamp,
 								Asset:     e.chain.UtilityTokenHash(),
 								Address:   "", // burn has empty receiver
-								Amount:    amountToString(int64(amount), 8),
+								Amount:    amountToString(big.NewInt(amount), 8),
 								Index:     b.Index,
 								TxHash:    b.Hash(),
 							})
@@ -282,7 +283,7 @@ var rpcTestCases = map[string][]rpcTestCase{
 							Timestamp: b.Timestamp,
 							Asset:     e.chain.UtilityTokenHash(),
 							Address:   "", // minted from network fees.
-							Amount:    amountToString(int64(netFee), 8),
+							Amount:    amountToString(big.NewInt(netFee), 8),
 							Index:     b.Index,
 							TxHash:    b.Hash(),
 						})