To elucidate the influence of methylprednisolone (MPL) on the antitumor effect of cis-diamminedichloroplatinum (II) (CDDP), experimental chemotherapy of CDDP with or without MPL was performed. A human breast carcinoma, MX-1 serially transplanted into nude mice was treated with CDDP in doses of 1.5 and 3.0 mg/kg in schedule of q4d X 3 ip. KKN-1, a syngeneic fibrosarcoma of BALB/c transplanted into BALB/c +/+ or BALB/c nu/nu mice, was treated with CDDP in doses of 3 or 6 mg/kg in schedule of qd X 1 ip. In the combination of MPL, 20 mg of MPL per kg was administered ip three hours before CDDP treatment. Although some decrease of antitumor effect of CDDP was observed by combination with MPL, no statistically significant difference was noted between group given CDDP alone and those given CDDP plus MPL. It was concluded that the use of MPL is reasonable to prevent the severe emesis caused by CDDP when this adverse effect is resistant to other antiemetic agents.