The in vitro effect of lentinan in inducing activation of killer cells and cytotoxic macrophages has been examined. Human peripheral blood mononuclear cells were cultured with lentinan for 2, 4 and 8 days. After 4 days cytotoxicity was increased 4% by lentinan less than 1,000 ng/ml. After 8 days, it was increased 12% by 25 and 1,000 ng/ml lentinan. The phenotype of the killer cells induced by lentinan was CD2+, CD16+ and CD56+, suggesting that they were natural killer cells. Macrophages separated from the spleens of 6 patients with gastric cancer were cultured with lentinan for 7 days, and their cytotoxicity increased 19%. The optimal concentration of lentinan was from 25 to 100 ng/ml. The findings suggest that the antitumour effect of lentinan is due to the activation of killer cells in vivo, because the optimal concentration of lentinan for the induction of killer cells in vitro was equivalent to the plasma concentration obtained after clinical doses of this agent.