The effect of postoperative immunochemotherapy with mitomycin C (MMC), 5-fluorouracil (5-FU) and OK-432 was evaluated as an adjuvant therapy after curative resection for gastric cancer. Immediately after surgery, patients were randomly allocated to the following three treatments: (A) chemotherapy with MMC and 5-FU (32 cases); (B) chemoimmunotherapy with MMC, 5-FU and OK-432 (33 cases); and (C) surgery alone as control (34 cases). There were no significant differences in the background factors influencing survival time among the groups, and there was no dose-distribution of chemotherapeutic agents between groups A and B. While the differences were not statistically significant, the survival rate and disease-free interval of group B were better than those of groups A or C. Side effects such as gastroenteric disorder, leukopenia (less than 3,000/mm3), thrombocytopenia (less than 7 X 10(4)/mm3) and increase of serum transaminase level (GPT greater than or equal to 100 units) were less frequently observed in group B than in group A. The results of the present study seemed to indicate that chemoimmunotherapy with OK-432 may be effective for surgical adjuvant therapy.