cis-Dichlorodiammineplatinum(II) (cis-platinum) has no more than additive, and often much less than additive, lethal toxicity for mice when given in combination with other anticancer agents representing several of the major functional classes of clinically useful anticancer drugs. The previously reported broad spectrum of anticancer activity of cis-platinum against tumors in laboratory animals has now been extended to promisingly useful therapeutic synergism in combination with other active anticancer drugs, including advanced-staged tumors in mice; eg, cis-platinum plus cyclophosphamide against advanced Ridgway osteogenic sarcoma and advanced P388 leukemia, and as surgical adjuvant chemotherapy against advanced colon tumor 26; cis-platinum plus Adriamycin against advanced P388; and cis-platinum plus VP-16-213 against advanced P388. Therapeutic synergism was also seen with cis-platinum plus carminomycin (an Adriamycin analog) against early colon tumor 26. Resistance and cross-resistance studies using sublines of L1210 and P388 selected for resistance to various alkylating agents (cyclophosphamide, melphalan, BCNU, or cis-platinum) indicate a variety of resistance and cross-resistance patterns which further support the growing body of evidence that wide differences in mechanism of cytotoxic activity exist among alkylating agents having experimentally and clinically useful anticancer activity. These data support the observed therapeutic synergisms with combinations of alkylating agents seen against a broad spectrum of murine tumors, and they suggest other drug combinations that might be considered for experimental and clinical trial based on a growing number of logical differences in biochemical mechanism of action of alkylating agent anticancer drugs that have been reported.