The effects of bone morphogenetic protein-2 (BMP-2) on osteoblastic differentiation of bone marrow stromal cells were investigated using two bone marrow stromal cell lines, ST2 and MC3T3-G2/PA6 (PA6). BMP-2 stimulated ALP activity and induced parathyroid hormone (PTH)-dependent production of cAMP in both ST2 and PA6 cells, but these effects were more apparent in ST2 cells than in PA6 cells. BMP-2 induced the production of osteocalcin in ST2 cells, but not in PA6 cells. BMP-4 and BMP-6 stimulated ALP activity in ST2 cells, but the effect of BMP-6 was less marked than that of BMP-2 and BMP-4. BMP-4 induced PTH-dependent cAMP production of cAMP in ST2 cells, but BMP-6 did not. When ST2 cells were transplanted into the peritoneal cavities of athymic mice with BMP-2 in diffusion chambers, these cells generated mineralized bone in the chambers. These results indicate that BMPs induce the differentiation of bone marrow stromal cells into osteoblasts. However, the effects differ among the BMPs and among the types of cell exposed to these proteins.