Lentinan could enhance the proliferative response of the mouse spleen lymphocytes to FHA, LPS ( Lipopolysaccharides ) and MLR ( Mixed Lymphocyte Reaction ) in vitro in a concentration-dependent manner. At the concentration of 25M-g/ml, it had the most prominent effects and the enhancing rates were 42.1%, 24.1% and 27.1% respectively. At much higher or lower concentrations, however, it had almost no effects. But lentinan at 25H-g/ml could inhibit the prolifera-tire response to Con A ( Concanavalin A ) and the inhibitory action increased with the increase in the concentration. At the concentration of 200 p-g/ml, the inhibition rate was 52.2%.