It is well known that the sodium thiosulfate (STS) decrease not only the nephrotoxicity which is a dose limiting factor of cisplatin (CDDP) chemotherapy but also its antitumor activity. Therefore, we examined the antitumor effect of CDDP in the human gastric cancer cell lines under various concentrations of STS. The cell lines used were KATO-III, MKN 28, MKN 45 and MKN 74. The consistency of STS in relation to CDDP (2.0 ng/ml) was 25-250 fold-molar ratio. The chemosensitivity of CDDP was positive in the 3 cell lines, KATO-III, MKN 28 and KATO-III in order. When STS was used with 50 fold-molar ratio in KATO-III and MKN 28 and 100 fold in MKN 45, a CDDP colony inhibition rate of more than 80% was maintained. This results suggest that the antitumor effect of CDDP will not be suppressed not so much when the low dose of STS was used in CDDP with STS chemotherapy, even though used through the same route of administration and simultaneously.