The antioxidative activity of a chymotrypsin‐specific potato type I inhibitor from Momordica cochinchinensis (MCoCI) (Cucurbitaceae) has been investigated using the primary rat hepatocyte system. tert‐Butyl hydroperoxide (t‐BHP) was used to induce oxidative stress. Pretreatment of hepatocytes with MCoCI for 24 h significantly reversed t‐BHP‐induced cell damage, and the associated glutathione depletion and lipid peroxidation. The activities of glutathione‐S‐transferase and superoxide dismutase were also increased. These results suggested that MCoCI possessed antioxidative activity which may account for some of the pharmacological effects of Momordica cochinchinensis seeds, the traditional Chinese medicine known as Mubiezhi, from which MCoCI was isolated. Copyright © 2005 European Peptide Society and John Wiley & Sons, Ltd.