Activin B has been reported to promote the regeneration of hair follicles during wound healing. However, its role in the development and life cycle of hair follicles has not been elucidated. In our study, the effect of activin B on mouse hair follicles of cultured and neonatal mouse skin was investigated. In these models, PBS or activin B (5, 10 or 50 ng/ml) was applied, and hair follicle development was monitored. Hair follicle initiation and development was examined using hematoxylin and eosin staining, alkaline phosphatase activity staining, Oil Red O+ staining, and the detection of TdT-mediated dUTP-biotin nick end-labeling cell apoptosis. Activin B was found to efficiently induce the initiation of hair follicles in the skin of both cultured and neonatal mice and to promote the development of hair follicles in neonatal mouse skin. Moreover, activin-B-treated hair follicles were observed to enter the anagen stage from the telogen stage and to remain in the anagen stage. These results demonstrate that activin B promotes the initiation and development of hair follicles in mice.