Arginase, a potent immune inhibitor, is studied in gastric cancer for its possible role in immunosuppression. Aqueous extracts (n = 12) from gastric cancer tissue inhibited PHA-induced lymphocyte proliferation more than those from normal gastric mucosal tissues (p less than 0.05). The inhibition was not due to cytotoxic effect of the extracts. Gastric arginase was isolated from gastric extracts by an affinity column conjugated with an anti-arginase antibody. The arginase from both normal and cancer gastric tissues strongly inhibited lymphocyte proliferation. By using enzyme-immunoassay, the arginase contents in cancer tissues were determined and showed 3 times as much as that in normal gastric mucosal tissues (n = 14) (p less than 0.005). The increased arginase content in cancer tissue was further confirmed by immunohistochemistry study. Arginase exists abundantly amounts in the cytoplasm of cancer cells, much more than that of normal cells (n = 30). It is conceivable that the depressed cellular immunity in gastric cancer patients might be partly due to the abundant amount of arginase in cancer cells.