Soybeans are the most common source of isoflavones in human food, and have been shown to have anti-inflammatory and anti-oxidant properties. We hypothesize that isoflavones modulate both Th1 and Th2 immune responses by regulating dendritic cell (DC) function. We have addressed this hypothesis with in vitro DC activation and coculture models, and in vivo models of lipopolysaccharide (LPS) and cholera toxin (CT)-induced sensitization to antigen. In vitro, we found significant suppressive effects of isoflavones on DC surface molecule expression and cytokine secretion, and subsequently, the ability of these DCs to induce T cell and natural killer (NK) cell activity. In vivo, we found that isoflavones cause significant suppression of LPS and CT-induced immunity, and that isoflavones may exert effects in their active forms at the site of sensitization, or as conjugates in blood. These results indicate that isoflavones may have a significant role in both Th1 and Th2-mediated sensitization.