The present study was undertaken to determine the adenosine deaminase (ADA) activity of peripheral lymphocytes in patients with gastric cancer, with respect to the cancer progression, the effect of surgery and/or immunotherapy. The gastric cancer patients showed lower lymphocyte ADA activity than did the normal control. The lymphocyte ADA activity did not decrease with the cancer progression. There was a significant correlation between lymphocyte ADA activity and blastogenesis of lymphocyte by phytohemaglutinin or concanavalin A. Six months following gastrectomy, the lymphocyte ADA activity was increased, as compared with the preoperative value. The ADA activity of patients on post-operative OK-432 showed a greater increase, as compared to that of patients not given this treatment. In conclusion, decreased lymphocyte ADA activity in gastric cancer patients might be due to either the cancer bearing status or to the immunological suppression.