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 and genistein, a prominent and more bioavailable isoflavone in soy foods, on murine immune function. Daidzein (50 mg/kg) or genistein (50 mg/kg) was administered p.o. once a day for 7 days in BALB/c mice. Daidzein decreased the mitogen-stimulated proliferation of murine splenocyte, but genistein increased it. Daidzein stimulated the secretion of interleukin-4, but inhibited the secretion of ${gamma}$-interferon, interleukin-2 and tumor necrosis factor-$alpha$. Genistein stimulated the secretion of ${gamma}$-interferon, interleukin-2 and tumor necrosis factor-$alpha$, but inhibited the secretion of interleukin-4. Daidzein and geiustein inhibited the production of nitric oxide and enhanced the phagocytic activity in peritoneal macrophage. These results suggest that cancer preventive effects of daidzein is partly concerned with the secretion of $T_{H}$2 cells cytokine and the activation of macrophage phagocytosis, and genistein is partly concerned with the secretion of $T_{H}$l cells cytokine, tumor necrosis factor-$alpha$ and the activation of macrophage phagocytosis.sis.