"One-way" mixed lymphocyte reaction was not enhanced when the cells were allowed to be exposed to acetylcholine or carbamylcholine at a concentration of 0.01-1.0mumole/ml during the entire period of culture. It was thought to be due to cytotoxicity effect of these agents on responding cells. When the responding cells were incubated with cholinergic agents for 1 hr prior to washing the cells and setting up the mixed cell culture, "one-way" mixed lymphocyte reaction was enhanced. However, when the stimulating cells were similarly treated with either agent, no enhancement of "one-way" mixed lymphocyte reaction was observed. These observations indicate that cholinergic agent induces enhancement of mixed lymphocyte reaction by affecting only the responding cells and not the stimulating cells.