The effect of intravenous administration of lentinan, an immunopotentiating polysaccharide, on the production of interleukin 1-alpha (IL 1-alpha), interleukin 1-beta (IL 1-beta) and tumor necrosis factor-alpha (TNF-alpha) by monocytes in peripheral blood mononuclear cells (PBM) was studied in patients with gastric carcinoma. Peripheral blood samples were obtained from 10 patients before and 3, 5 and 7 days after a single dose of 2 mg lentinan injection. The ability of monocytes in PBM to produce IL 1-alpha was significantly augmented 3 and 5 days after lentinan administration, as compared with that before treatment. IL 1-beta production was also significantly increased 3, 5 and 7 days after the drug injection. Further, the capacity to produce TNF-alpha was significantly enhanced 3, 5 and 7 days after the drug administration. Thus, it is likely that the augmentation of these cytokine's production may contribute to the antitumor action of lentinan in patients with gastric carcinoma.