The therapeutic antitumour activity and host toxicity of cis-platin (CDDP), which was administered with selenium (sodium selenite) was studied on the growth of a human yolk sac tumour grown in nude mice. Treatment consisted of CDDP single agent chemotherapy (3 weeks) or preliminary PVB combination chemotherapy (CDDP + vinblastine + bleomycin, 2 weeks). Selenium was co-administered from day 1 to 5 with each therapeutic regimen. The administration of CDDP alone caused significant reduction in tumour burden but at higher doses there was significant host toxicity. The co-administration of selenium together with CDDP (CDDP: selenium, molar ratio = 3.5:1) did not affect the anti-tumour activity of CDDP but it did cause a decrease of parameters of host toxicity including lethality, increasing the 50% lethal dose (LD50) from 9.3 mg kg-1 to 17.5 mg kg-1. The parameters of host toxicity which were altered by selenium co-administration were nephrotoxicity, myeloid suppression and weight loss. Our study suggested that selenium co-administration allows higher doses of CDDP with reduction of apparent toxicity, resulting in a higher therapeutic index and possibly indicating a potential increase in the utilization of CDDP in clinical cancer chemotherapy.