Our previous study showed the inhibitory effect of Qing-Fei-Tang (Q.F.T.) and baicalein on the leukotriene (LT)B4 synthesis of human alveolar macrophages. It has recently been demonstrated that LTs support various cell growth, and basophil and its precursor numbers increase in atopic patients. Therefore, we examined the effect of anti-allergic drugs, including Q.F.T., Xiao-Qing-Long-Tang (X.Q.L.T.), Chai-Pu-Tang (C.P.T.), baicalein and ketotifen which have been used for treatment of bronchial asthma, on human basophil growth in vitro using cord blood mononuclear cells as a basophil precursor source and conditioned medium of T cell leukemia cell line Mo as a growth factor. Two-week cultured basophil numbers identified by alcian blue-safranin staining and those histamine contents assayed fluorometrically were inhibited by Q.F.T. (1.0 mg/ml), X.Q.L.T. (0.01-1.0 mg/ml), C.P.T. (0.01-1.0 mg/ml), baicalein (1-100 microM) or ketotifen (1-100 microM) in a dose-dependent manner while low dose (0.01-0.1 mg/ml) of Q.F.T. showed an enhancing effect on the basophil growth and the histamine content. However, LTB4 or LTC4 failed in restoring the basophil growth reduced by 1 mg/ml of C.P.T. or 100 microM of ketotifen. These results suggest that anti-allergic drugs may modulate basophil growth and differentiation in vitro and/or in vivo and therefore be useful and reasonable for controlling allergic diseases including bronchial asthma.