The current study was designed to compare the effects of oral administration of the streptococcal preparation, OK-432, as an adjuvant immunotherapy versus those of intradermal administration of OK-432 on the survival of patients after surgery for gastric cancer. The patients were stratified into two groups after surgery: a curative surgery stratum and a palliative surgery stratum. Then the patients in each stratum were randomly assigned into three groups: an oral placebo group, an oral OK-432 group, and an intradermal OK-432 group. All of the patients were given fluoropyrimidines orally in combination with OK-432 or placebo for 2 years after surgery. A total of 1011 patients were registered between 1982 and 1985, and 970 patients were eligible for statistical analysis. The survival rate of the oral OK-432 group was significantly higher than those of the other two groups after curative surgery. There were no significant difference in the survival rates between the three groups after palliative surgery, however. The effect of oral OK-432 was quite pronounced in patients after curative surgery for stage II to IV gastric cancer, especially in those patients with regional node involvement. Furthermore, it was found that the spleen is necessary for effective immunotherapy with oral OK-432, because the survival rate of the oral OK-432 group was significantly improved in patients whose spleens were preserved, when compared with splenectomized patients. These results demonstrate that oral adjuvant immunotherapy with OK-432 is beneficial after curative surgery for gastric cancer.