The inhibitory effect of dl-puerol A as but-2-enolide isolated from Sophora japonica was investigated on copper ion-induced protein oxidative modification in vitro. It inhibited copper-induced protein oxidative modification. However, its inhibitory effect was a little weaker than that of $dl-{alpha}-tocopherol$ as an antioxidant. The results demonstrated that dl-puerol A, one of but-2-enolides, might be of use in the oxidative stress.