Three metabolites having a suppressive effect on proliferation of mouse lymphocytes stimulated with mitogens such as concanavalin A (Con A) and lipopolysaccharide (LPS) were isolated from a mushroom, Lactarius flavidulus, and identified as geranylphenols, flavidulols A, B and C, respectively, which had previously been isolated from the same mushroom. The IC₅₀ values of flavidulols A, B and C were found to be 8.9, 4.9 and 36.3 μg/ml against Con A-induced proliferation of mouse lymphocytes, and 6.7, 3.9 and 28.3 μg/ml against LPS-induced proliferation, respectively.