Several factors increase oxidation in post-mortem meat. One of these factors is irradiation. Vitamin E supplementation reduces the amount of oxidation in postmortem (PM) meat tissue through its' antioxidant properties. The hypothesis was oxidative conditions in PM tissue would decrease calpain activity and minimize tenderization. Ten beef steers were fed a finishing diet that included 1000 IU per head/per day of vitamin E. Another ten beef steers were fed the same finishing diet without supplemental vitamin E. Steers were harvested after a minimum of 126 days on feed. At 24 h PM, boneless longissimus dorsi et lumborum (LDL) from each animal were cut into 2.54 cm steaks and individually vacuum packaged. One steak was taken from each animal to determine LDL vitamin E content. Steaks were randomly assigned to either an irradiated or non-irradiated control group. Steaks were aged at 40C for 0, 1, 3, 7, and 14 d post-irradiation. Steaks from each time point were used to determine Warner-Bratzler shear force (WBS), carbonyl content, sulfhydryl content, protein degradation, calpain activity, and calpain autolysis. Steaks from animals supplemented with vitamin E had significantly higher α-tocopherol content than steaks from animals not supplemented with vitamin E (P<0.0001). At 1, 3, 7, and 14 d post-irradiation, WBS values of irradiated steaks were significantly higher compared to non-irradiated steaks (P<0.05). Carbonyl content of the supernatant fraction and highly purified myofibrils was significantly higher in irradiated steaks compared to non-irradiated steaks at 0, 1, 3, and 7 d post-irradiation. At d 0 post-irradiation, sulfhydryl content of highly purified myofibrils was significantly lower in irradiated steaks compared to non-irradiated steaks (P<0.05). Less proteolysis of troponin-T and desmin was seen in irradiated samples compared to non-irradiated samples. Also, at 2 d post-mortem more troponin-T degradation was observed in non-irradiated steaks from animals supplemented with vitamin E than non-irradiated steaks from animals fed the control diet (P<0.05). Casein zymography and western blotting revealed [Mu]-calpain had less activity and slower autolysis in irradiated samples compared to non-irradiated samples. Loss of [Mu]-calpain activity resulted in decreased myofibrillar proteolysis subsequently minimizing the extent of tenderization in beef strip-loin steaks.