The enhanced effects due to the combined use of oral administration of S-1 and intraperitoneal administration of Cisplatin (CDDP) were examined with gastric cancer xenografts (NUGC 4). S-1, a new anticancer drug, was daily administered at 10 mg/kg (qld x 5 x 3 weeks). 5-FU level in blood was 1 microgram/ml at two hours after the treatment. Antitumor activity was not found in mice with only the CDDP treatment. But antitumor activity by S-1 and daily low-dose (1 mg/kg) or intermittent treatment (5 mg/kg) of CDDP showed better results than daily S-1 treatment. The daily low-dose CDDP treatment showed similar efficacy to the intermittent administration at the same total dose, but the daily low-dose CDDP treatment was better in the light of toxicities. These results suggest that treatment with S-1 and daily low-dose CDDP was effective for gastric cancer.