Lymphokine activated killer (LAK) cells derived from normal subjects were examined as to the following points: 1) monoclonal analysis of LAK, 2) cytotoxic ability of LAK, 3) effect on LAK cytotoxic ability of the presence in the medium of either serum obtained from gastric cancer patients or nonspecific immunosuppressive factors (ferritin, IAP, AFP), 4) effect, on induction of their cytotoxicity, of the presence in the medium during culture of sera from gastric cancer patients, simulating the conditions of in vivo administration and 5) augmentation of cytotoxic ability of LAK by simultaneous IL2 administration. The following results were obtained. 1) Monoclonal marker analysis of LAK revealed that the ratios of OKT3+, OKT4+, OKT8+ and OKIa1+ lymphocytes were all significantly higher than those in peripheral blood lymphocytes (PBL). 2) Cytotoxic ability of LAK against various tumor cell lines (MKN-28, MKN-45, KATOIII, PC-10 and K562) was found to be higher than that of PBL. 3) Addition to medium of ferritin, IAP or AFP significantly reduced the cytotoxic ability of both PBL and LAK against various tumor cell lines. However, the degree of reduction was significantly milder in the case of LAK than in PBL. 4) The cytotoxicity-suppressing effects of gastric cancer sera (untreated, at stages III and IV) were significantly milder in the case of LAK than in PBL. 5) When gastric cancer serum was added to medium, instead of normal AB serum during induction of LAK, its cytotoxic ability against various tumor cell lines was significantly reduced. Its cytotoxic ability was nevertheless significantly higher than that of PBL. 6) When IL2 was added to medium during cytotoxicity assay, cytotoxic ability of LAK was augmented. When LAK was cultured for 1 hour before assay in fresh medium containing 1,000 U/ml IL2, its cytotoxic ability was further augmented.