In order to augment antitumor immunity of gut-associated lymphoid tissue in digestive organ cancer, oral administration of various biological response modifiers were studied using mice transplanted with cecal tumor. Among them OK-432 was the most effective and clinical application of oral OK-432 was studied. Autoradiogram and immunofluorescence studies showed the absorption of orally administered OK-432 from the gut. In phase I study oral OK-432 was much less toxic than other administration routes. Phase II study showed that oral OK-432 at various doses augmented antitumor immunity of the lymphocytes of peripheral blood and regional lymph node. In a multi-institutional study on postoperative adjuvant immunotherapy, 1011 gastric cancer patients were accumulated and randomized to compare the effects of oral and intradermal OK-432. In patients who underwent curative operation, 1-, 2- and 3-year survival rates (%) were 95, 88 and 82 for oral OK-432 group (n = 255), 88, 83 and 80 for the placebo group (n = 260), and 89, 79 and 73 for intradermal OK-432 group (n = 261). There was significant differences among cumulative survivals of three groups, and this life-prolonging effect of oral OK-432 was remarkable for stage II or III patients. These results demonstrate that oral immunotherapy with OK-432 is useful as an immunotherapy of digestive organ cancers.