diff --git a/pkg/core/native/std.go b/pkg/core/native/std.go
index 26fc49115..04211b59a 100644
--- a/pkg/core/native/std.go
+++ b/pkg/core/native/std.go
@@ -238,7 +238,6 @@ func (s *Std) itoa(_ *interop.Context, args []stackitem.Item) stackitem.Item {
 		if pad := bs[0] & 0xF8; pad == 0 || pad == 0xF8 {
 			str = str[1:]
 		}
-		str = strings.ToUpper(str)
 	default:
 		panic(ErrInvalidBase)
 	}
diff --git a/pkg/core/native/std_test.go b/pkg/core/native/std_test.go
index 2817850e0..da56a4f90 100644
--- a/pkg/core/native/std_test.go
+++ b/pkg/core/native/std_test.go
@@ -35,9 +35,9 @@ func TestStdLibItoaAtoi(t *testing.T) {
 			{big.NewInt(1), big.NewInt(16), "1"},
 			{big.NewInt(7), big.NewInt(16), "7"},
 			{big.NewInt(8), big.NewInt(16), "08"},
-			{big.NewInt(65535), big.NewInt(16), "0FFFF"},
-			{big.NewInt(15), big.NewInt(16), "0F"},
-			{big.NewInt(-1), big.NewInt(16), "F"},
+			{big.NewInt(65535), big.NewInt(16), "0ffff"},
+			{big.NewInt(15), big.NewInt(16), "0f"},
+			{big.NewInt(-1), big.NewInt(16), "f"},
 		}
 
 		for _, tc := range testCases {