Tylogenin, a steroidal aglycone generated by acid hydrolysis from two seasonal glycosides occurring in Tylophora sylvatica, inhibits IgE-induced basophil mediator release for allergic reactions. In the rabbit basophil-dependent serotonin release (BDSR) assay system, the inhibitory activity of tylogenin was significantly greater than that of its parent glycosides, tylophoroside and acetyltylophoroside, and that of dexamethasone. The activity of tylogenin was found to increase with the incubation time. In the human leukocyte-dependent histamine release test model, the glycosides had only a minimal activity. In contrast, tylogenin, with a geom mean IC50 = 49 microM, exerted a significantly greater potency than dexamethasone. These results suggest that tylogenin could represent a new class of antiallergic agents.