We have demonstrated that the proliferation of estrogen-responsive mouse Leydig tumor cell line B-1F is induced via suppression of 5-lipoxygenase activity followed by decrease of leukotrienes (LTs). Additionally, it has been reported that LTD4 induces apoptosis in B-1F cells. In this study, we examined effects of Saiboku-to, a traditional Chinese medicine having suppressive activities for LT production and release, on the proliferation. Saiboku-to promoted, but Scutellaria baicalensis, one of components (herbs) of Saiboku-to, significantly inhibited the proliferation of B-1F cells in vitro and in vivo. The action of Scutellaria baicalensis in B-1F cells was studied in more detail. Although Scutellaria baicalensis consists of flavonoids, iridoids, volatile oils and others, it and its major constituents had no direct effect on estrogen binding sites in B-1F cells. B-1F cells treated with Scutellaria baicalensis showed morphological changes such as nuclear aggregation and fragmentation. DNA fragmentation was also observed, indicating that Scutellaria baicalensis induces apoptosis in B-1F cells and that it or its constituents might be a good resource for searching new drugs, especially anti-cancer drugs. Moreover, Saiboku-to promoted B-1F cell proliferation, but Scutellaria baicalensis inhibited it, showing complexity of action of traditional Chinese medicines.