Production of a member of transforming growth factor-beta (TGF-beta) superfamily, activin A, was examined in the bone tissue by using reverse transcriptase polymerase reaction. As a result, specific bands were detected showing the presence of activin A mRNA in the bone tissues. In order to localize the production site of activin A in the bone tissues, we tried to immunolocalize activin A in fetal mouse calvaria cultured in a medium containing fetal calf serum, 1 alpha-25(OH)2 vitamin D2 and parathyroid hormone. In these cultured calvaria, bone tissues including bone-resorbing osteoclasts in vitro were observed. Positive staining demonstrating the presence of activin A resided inside of the multinucleated cells in the bone resorbing lacunae, suggesting the production of activin A in osteoclasts. Activin A was also localized immunohistochemically in the osteoclast-like multinucleated cells developed in vitro. These results suggest that osteoclast produce activin in the bone tissues and that activin may play some roles by autocrine and/or paracrine manner in bone metabolisms.