The immunopotentiator "Lentinan" augmented the cell-mediated cytotoxicity in humans. (a) The activation of killer T cells by the mixed lymphocyte culture was accelerated only when responder cells were mixed with both a suboptimum number of stimulator cells and Lentinan. (b) The interferon level in the peripheral blood circulation of cancer patients was elevated in 12 h following Lentinan administration, and natural killer activity of peripheral mononuclear cells was enhanced in 48 h. These data indicate that Lentinan favorably affects the host mechanisms of man.