Antitumor effects of carboplatin and iproplatin, a second-generation cisplatin analog were studied using cisplatin sensitive human urinary bladder (NM-B-1) and prostatic cancers (PRO-1) grown in nude mice. Both NM-B-1 and PRO-1 were sensitive to carboplatin and iproplatin. The tumor-regression effect of carboplatin at the fourfold dose of cisplatin was comparable to that of cisplatin. Iproplatin at the eight to sixteen times dose of cisplatin showed a comparable tumor regression to cisplatin. The range of effective dose was wider in carboplatin than cisplatin and that of iproplatin was not so wide as cisplatin.