Saiboku-to is a useful drug in the treatment of intractable asthmatics. The most important action mechanism of Saiboku-to seems to be a suppression on lymphocyte activation. The relationship between the in vitro and in vivo effects is not clear because the metabolism of Kampo medicine is unknown. Therefore, in this study two in vitro experimental systems were compared to evaluate the pharmacological effect of Saiboku-to on the lymphocyte blas-togenesis. The first system involved the addition of serum obtained from guinea pigs treated orally with Saiboku-to, while the second system involved the direct additon of the Saiboku-to extract to the medium. Saiboku-to has a suppressive effect on lymphocyte blastogenesis and also the metabolite of Saiboku-to had a suppressive effect. According to the formula based on these data, the serum level of Saiboku-to on oral administration of the common dose was assumed to be 5.65μg/ml in bronchial asthmatics. These findings suggest that Saiboku-to may an inhibitory effect in the cell-mediated allergic reaction by lymohocyte activation in vivo.