The immunosuppressive properties of flavonoids were examined for the first time by testing their effects on T-cell-mediated antibody production, using a classical plague-forming cell (PFC) assay in mice. Among the tested flavonoids including naringenin, chrysin, flavonol, galangin, quercetin, morin, myricetin and biochanin A, only quercetin, orally administered at 25 mg/kg, significantly inhibited the number of IgMproducing PFCs induced by sheep red blood cells (SRBC). Interestingly, biochanin A (isoflavone) increased the number of PFCs, suggesting an immunostimulatory effect. The other flavonoids tested did not inhibit or enhance PFC response significantly. Quercetin was also found to show thymus atrophy dose-dependently at 5-500 mg/kg. All these results indicate that quercetin inhibits in vivo antibody production probably by inhibiting T-cell function.