We have previously shown that the combined modalities of reovirus type 3 and the chemotherapeutic agent 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU) synergize to cause the rejection of various murine tumors. Resistance of surviving animals to challenge with homologous but not heterologous tumor suggested that tumor was eliminated through an immune-mediated mechanism. In this study, we showed that mice undergoing therapy-mediated rejection of tumor were able to reject subsequent weekly challenges with EL-4 but not L1210 tumor cells. The mechanism underlying this therapy was investigated using cyclosporine (CS) to suppress immune responsiveness. A dose-related inhibition of therapy was observed with total inhibition occurring at 30 mg/kg/day. Delayed administration of CS at day 14 or later after tumor administration resulted in little or no inhibitory effect. The resistance of cured mice to EL-4 tumor challenge was not affected by CS, which is consistent with the reduced ability of CS to affect secondary immune responses. In addition, CS did not alter natural killer cell activity in mice receiving the BCNU/reovirus therapy. These results suggest that there is an obligatory immune response produced by the BCNU/reovirus therapy which arises early after the administration of the therapy.