The antitumor activity of [SP-4-3-(R)]-[1,1-cyclobutanedicarboxylato-(2-)](2 methyl-1,4-butanediamine-N,N′)platinum (CI-973) was characterized in a number of preclinical model systems. CI-973 retained substantial activity in cisplatin resistant murine leukemia cell lines, in vitro and in vivo; in L1210 leukemia resistant to cisplatin in vivo, CI-973 retained as much activity as was found in animals bearing sensitive L1210 leukemia. When compared in five solid murine tumors in vivo, both CI-973 and cisplatin were approximately equivalent in activity. In one human colon tumor and one human non-small cell lung carcinoma tested as xenografts in immunodeficient mice, cisplatin and CI-973 were ineffective. In two other human non-small cell lung carcinomas tested in the same fashion, cisplatin did possess activity. CI-973 has entered phase I clinical trials.