The present study was designed to examine the antitumor efficacy of combination chemotherapy with sodium thiosulfate (STS) and cis-diamminedichloroplatinum (CDDP) using a murine model. Xenograft tumors of the endometrial adenocarcinoma cell line responded well to intraperitoneal chemotherapy with CDDP in nude mice. However, addition of STS to CDDP treatment canceled the antitumor effect of CDDP. Such a finding was obtained in mice injected subcutaneously with STS even 72 hr after treatment with CDDP. On the other hand, alleviation of CDDP-induced side effects by administration of STS was not observed in mice treated by several modalities of combination chemotherapy with STS and CDDP. Thus, addition of STS to CDDP treatment reduced CDDP's antitumor efficacy with only minimal alleviation of side effects, suggesting that it would provide no benefit in patients treated with CDDP.