diff --git a/evm/src/cpu/kernel/tests/exp.rs b/evm/src/cpu/kernel/tests/exp.rs
index 388a1ac3..25bc5ad3 100644
--- a/evm/src/cpu/kernel/tests/exp.rs
+++ b/evm/src/cpu/kernel/tests/exp.rs
@@ -1,5 +1,3 @@
-use std::str::FromStr;
-
 use anyhow::Result;
 use ethereum_types::U256;
 use rand::{thread_rng, Rng};
@@ -17,7 +15,7 @@ fn test_exp() -> Result<()> {
     let b = U256([0; 4].map(|_| rng.gen()));
 
     // Random input
-    let initial_stack = vec![U256::from_str("0xdeadbeef")?, b, a];
+    let initial_stack = vec![0xDEADBEEFu32.into(), b, a];
     let stack_with_kernel = run_with_kernel(&kernel, exp, initial_stack)?
         .stack()
         .to_vec();
@@ -29,7 +27,7 @@ fn test_exp() -> Result<()> {
     assert_eq!(stack_with_kernel, stack_with_opcode);
 
     // 0 base
-    let initial_stack = vec![U256::from_str("0xdeadbeef")?, b, U256::zero()];
+    let initial_stack = vec![0xDEADBEEFu32.into(), b, U256::zero()];
     let stack_with_kernel = run_with_kernel(&kernel, exp, initial_stack)?
         .stack()
         .to_vec();
@@ -41,7 +39,7 @@ fn test_exp() -> Result<()> {
     assert_eq!(stack_with_kernel, stack_with_opcode);
 
     // 0 exponent
-    let initial_stack = vec![U256::from_str("0xdeadbeef")?, U256::zero(), a];
+    let initial_stack = vec![0xDEADBEEFu32.into(), U256::zero(), a];
     let stack_with_kernel = run_with_kernel(&kernel, exp, initial_stack)?
         .stack()
         .to_vec();
diff --git a/evm/src/cpu/kernel/tests/rlp.rs b/evm/src/cpu/kernel/tests/rlp.rs
index e74213da..79407f1b 100644
--- a/evm/src/cpu/kernel/tests/rlp.rs
+++ b/evm/src/cpu/kernel/tests/rlp.rs
@@ -1,5 +1,3 @@
-use std::str::FromStr;
-
 use anyhow::Result;
 use ethereum_types::U256;
 
@@ -11,7 +9,7 @@ use crate::memory::segments::Segment;
 fn test_decode_rlp_string_len_short() -> Result<()> {
     let decode_rlp_string_len = KERNEL.global_labels["decode_rlp_string_len"];
 
-    let initial_stack = vec![U256::from_str("0xdeadbeef")?, 2.into()];
+    let initial_stack = vec![0xDEADBEEFu32.into(), 2.into()];
     let mut interpreter = Interpreter::new_with_kernel(decode_rlp_string_len, initial_stack);
 
     // A couple dummy bytes, followed by "0x70" which is its own encoding.
@@ -28,7 +26,7 @@ fn test_decode_rlp_string_len_short() -> Result<()> {
 fn test_decode_rlp_string_len_medium() -> Result<()> {
     let decode_rlp_string_len = KERNEL.global_labels["decode_rlp_string_len"];
 
-    let initial_stack = vec![U256::from_str("0xdeadbeef")?, 2.into()];
+    let initial_stack = vec![0xDEADBEEFu32.into(), 2.into()];
     let mut interpreter = Interpreter::new_with_kernel(decode_rlp_string_len, initial_stack);
 
     // A couple dummy bytes, followed by the RLP encoding of "1 2 3 4 5".
@@ -45,7 +43,7 @@ fn test_decode_rlp_string_len_medium() -> Result<()> {
 fn test_decode_rlp_string_len_long() -> Result<()> {
     let decode_rlp_string_len = KERNEL.global_labels["decode_rlp_string_len"];
 
-    let initial_stack = vec![U256::from_str("0xdeadbeef")?, 2.into()];
+    let initial_stack = vec![0xDEADBEEFu32.into(), 2.into()];
     let mut interpreter = Interpreter::new_with_kernel(decode_rlp_string_len, initial_stack);
 
     // The RLP encoding of the string "1 2 3 ... 56".
@@ -69,7 +67,7 @@ fn test_decode_rlp_string_len_long() -> Result<()> {
 fn test_decode_rlp_list_len_short() -> Result<()> {
     let decode_rlp_list_len = KERNEL.global_labels["decode_rlp_list_len"];
 
-    let initial_stack = vec![U256::from_str("0xdeadbeef")?, 0.into()];
+    let initial_stack = vec![0xDEADBEEFu32.into(), 0.into()];
     let mut interpreter = Interpreter::new_with_kernel(decode_rlp_list_len, initial_stack);
 
     // The RLP encoding of [1, 2, [3, 4]].
@@ -86,7 +84,7 @@ fn test_decode_rlp_list_len_short() -> Result<()> {
 fn test_decode_rlp_list_len_long() -> Result<()> {
     let decode_rlp_list_len = KERNEL.global_labels["decode_rlp_list_len"];
 
-    let initial_stack = vec![U256::from_str("0xdeadbeef")?, 0.into()];
+    let initial_stack = vec![0xDEADBEEFu32.into(), 0.into()];
     let mut interpreter = Interpreter::new_with_kernel(decode_rlp_list_len, initial_stack);
 
     // The RLP encoding of [1, ..., 56].
@@ -110,7 +108,7 @@ fn test_decode_rlp_list_len_long() -> Result<()> {
 fn test_decode_rlp_scalar() -> Result<()> {
     let decode_rlp_scalar = KERNEL.global_labels["decode_rlp_scalar"];
 
-    let initial_stack = vec![U256::from_str("0xdeadbeef")?, 0.into()];
+    let initial_stack = vec![0xDEADBEEFu32.into(), 0.into()];
     let mut interpreter = Interpreter::new_with_kernel(decode_rlp_scalar, initial_stack);
 
     // The RLP encoding of "12 34 56".