Chemotherapy sometimes results in induction of specific antitumor immunity. We investigated the mechanisms responsible for the induction of antitumor immunity in mice bearing MOPC-104E plasmacytoma after chemotherapy with cisplatin (CDDP), especially the effects of CDDP on the expression of MHC on the tumor surface. BALB/c mice were subcutaneously (s.c.) inoculated with MOPC-104E cells on day 0, then intravenously (i.v.) treated with CDDP at 3.6 mg/kg on day 7. The tumors disappeared completely on day 35 and the mice rejected a second challenge with MOPC-104E, but did not reject syngeneic Meth-A fibrosarcoma. The tumors did not regress, however, when MOPC-104E was s.c. transplanted in nude mice, or when anti-T-cell monoclonal antibodies were i.v. injected into BALB/c mice before CDDP treatment on day 6. To determine which of the mice or tumor were affected by CDDP, BALB/c mice were inoculated with CDDP-treated (12.5 micrograms/ml for 3 hours in vitro) MOPC-104E cells on day 0, 7 and 14. The potential to reject MOPC-104E was lower in mice immunized with ethanol-treated MOPC-104E cells than in those immunized with CDDP-treated cells. CDDP-treated or -untreated MOPC-104E cells ultrasonicated and fractionated into soluble and insoluble fractions by centrifuging, also induced antitumor immunity. Flow cytometry demonstrated that the expression of MHC-class-I antigens H-2Dd and H-2Kd was enhanced after CDDP-treatment, but that of class-II antigens I-Ad and I-Ed was not, suggesting that CDDP induced tumor-specific antitumor immunity by enhancing the expression of MHC-class-I antigens.