The effect of WK101 on compound 48/80-induced anaphylaxis was studied in rat. WK101 was found to exhibit a inhibitory activity on the compound 48/80-induced anaphylaxis. WK101 also inhibited the serum histamine release induced in anaphylaxis by compound 48/80. The effect of WK101 on the histamine release from rat peritoneal mast cells was studied. WK101 ($10^{3}-1mg/ml$) inhibited the histamine release induced by compound 48/80($5{;}mu extrm{g}/ml$) in rat peritoneal mast cells. To clarify the mechanism of these inhibitons, we investigated the effects of WK101 on cAMP and intracellular calcium content of rat peritoneal mast cell. The content of cAMP in mast cells, when WK101 was added, was increased transiently, and was significantly increased more 53 fold at 10 sec than that of basal cells. Moreover, WK101 inhibited intracellular calcium release induced by compoound 48/80. This results suggest that WK101 may be useful for the prevention and treatment of allergy-related disease.