We studied the effects on survival time of postoperative immuno-chemotherapy, including the streptococcal preparation OK-432, in patients with gastric cancer and synchronous peritoneal dissemination. The patients were prospectively randomized and a valid statistical assessment could be made for 109. Patients randomized to group B received therapy that is widely used in Japan to treat patients with gastric cancer: mitomycin C (MMC) and UFT, a combination of tegafur and uracil in a molar ratio of 1:4, for 1 year. Patients randomized to group A received the same drugs as were given to group B patients plus OK-432 i.p. for 7 days, beginning on postoperative day 0, and OK-432 by intradermal injection for 1 year, at 2-week intervals. There were no differences between the two groups in any known prognostic factor or in the dose of any drug administered except for OK-432. There was no difference in the toxicity rate between the groups. In this negative trial, there was no improvement in survival time with the addition of OK-432 to MMC and UFT for patients with gastric cancer and peritoneal dissemination.