The effectors of cell death in allograft rejection are poorly understood. Oxygen derived free radicals (ODFR) may participate in graft destruction. We examined the impact of the antioxidants ascorbic acid (AA) and α-tocopherol (AT) with low dose CsA on rat cardiac allograft survival. Lewis rats that had undergone het-erotopic abdominal cardiac transplantation with Wis-tar-Furth allografts (day 0) were divided into 6 groups. Group 1 was the control group; groups 2 and 3 received AA (1200 mg/kg), and groups 4 and 5 received AT (800 IU/kg) by gavage daily until rejection. Groups 3, 5, and 6 were given CsA (2.5 mg/kg i.m.) days 1–15. Allograft rejection times (in days) were 7.7+1, 10.3+1.5 (P<0.01 vs. group 1), 37.1±6.4 (P<0.01 vs. group 1, P=0.0004 vs. group 6), 9.0±1.4, 26.5±3.6 (P<0.01 vs. group 1, P<0.03 vs. group 6), and 20±4.9 (P<0.01 vs. group 1) for groups 1, 2, 3, 4, 5, and 6. To assess the impact of AA on ODFR production, chemiluminescence was performed on zymosan-activated Lewis whole blood from control rats and rats administered AA. AA significantly decreased peak chemiluminescence (JP<0.05) as compared with nontreated rats indicating effective ODFR scavenging. To determine whether AA and AT inhibit lymphocyte stimulation, mixed lymphocyte response testing was performed with irradiated Wistar-Furth lymphocytes as stimulator cells for Lew responder cells from rats treated as groups 3, 5, and 6. CsA significantly suppressed (P<.05) proliferation as compared with untreated controls. Neither AA nor AT enhance CsA's immunosuppressive effect by mixed lymphocyte response testing. In summary, prolongation of allograft survival with antioxidants AA and AT does not result from abrogation of lymphocyte responsiveness or alteration in CsA bioavailability. Rather, these data suggest that ODFR are involved in allograft destruction and support a role for effective antioxidant therapy in the treatment of allograft rejection.