High soy consumption leading to high exposures of soy isoflavones has been associated with a reduced risk of cancers at many sites. As part of a study focusing on the chemopreventive mechanisms, we have investigated the modulating effects of daidzein, a prominent and more bioavailable isoflavone in soy foods, on murine immune function. Swiss mice were fed daidzein at various doses daily for seven consecutive days. At high doses (20 and 40 mg/kg), daidzein exerted a stimulatory effect on nonspecific immunity, as shown by increases in the phagocytic response of peritoneal macrophages and thymus weight, in a dose-dependent manner. Augmentation of spleen immunoglobulin M-producing cells against sheep red blood cells demonstrated an-activation of humoral immunity. Enhanced cell-mediated immunity was also observed as increases in lymphocyte proportion of peripheral blood. However, no significant immunoregulatory effect was found when mice were fed 10 mg/kg daidzein. These results demonstrate for the first time that daidzein at high doses enhances several immunologic functions and suggest a novel approach to understanding the mechanism(s) by which soy foods may contribute to observed cancer prevention.