The authors investigated K cell population and lymphocyte subpopulation in peripheral blood from 17 cases of gastric ulcer, 33 cases of gastric carcinoma and 34 cases of adult healthy control. Diagnosis were confirmed histopathologically in all the patients with gastric ulcer and carcinoma. Blood samples were taken from the anterior cubital vein of all the patients and control in Pusan National University Hospital during the period from January, 1984 to August, 1985. K cell population was examined by microtest plate assay with sheep erythrocytes as target cells. Lymphocyte subpopulation was examined by E rosette method neuramidase treated sheep erythrocytes for T cell and EAC rosette method for B cells. The results were summarized as follows: 1) K cell population and the percentage and count of T cells showed a significant decrease in gastric carcinoma, as compared with those in control and gastric ulcer. 2) K cell population tended to be higher in papillary adenocarcinoma than those in tubular adenocarcinoma and signet ring cell carcinoma of stomach. 3) Percentage and count of other lymphocytes showed a significant increase as compared with those in control and gastric ulcer. 4) No significant correlation between K cell population and lymphocyte subpopulation could be observed in gastric carcinoma. On the basis of these results, it was suggested that reduction of K cells in peripheral blood in gastric carcinoma may be induced by different mechanism and the assay of K cells show an important significance as immunological parameter in gastric carcinoma.