1. Effects of a naturally occurring flavonoid, silybin, on histamine release from human basophils were examined, in order to assess the potential utility in the treatment of allergic disorders. 2. The f-met peptide and anti-IgE-induced histamine release was significantly (P less than 0.05) inhibited in a concentration-dependent fashion. Conversely, no significant (P greater than 0.05) effect on calcium ionophore A23187-induced histamine secretion was documented. The inhibitory activity was significantly (P less than 0.05) reversed by elevating extracellular calcium concentrations. 3. The anti-allergic properties of silybin can be reasonably ascribed to a membrane-stabilizing activity, possibly related to an interference in calcium influx. These results indicate that an in vivo evaluation of the anti-allergic activity of silybin would be worthwhile.