Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta superfamily, which regulate the differentiation of osteoprogenitor cells. Here we show that among members of the BMP family, BMP-4 and growth/differentiation factor 5 (GDF-5) induce osteoblast differentiation through the activation of three receptor-regulated Smads (i.e. Smad1, Smad5 and Smad8). By contrast, BMP-6 and BMP-7 induce alkaline phosphatase activity through Smad1 and Smad5, but not through Smad8. Consistent with these findings, BMP-4 induced phosphorylation and nuclear translocation of Smad1, Smad5 and Smad8, but BMP-6 activated only Smad1 and Smad5. BMP-4 and GDF-5 are known to bind to activin receptor-like kinase 3 (ALK-3) and/or ALK-6 (also termed BMP type IA and type IB receptors, respectively), whereas BMP-6 and BMP-7 preferentially bind to ALK-2. Compared with the effects induced by only one of the type I receptors, the combination of constitutively active forms of ALK-2 and ALK-3 (or ALK-6) more strongly induced alkaline phosphatase activity in C2C12 cells. Moreover, addition of BMP-4 and BMP-6 to C2C12 cells resulted in higher alkaline phosphatase activity than that of only one of these BMPs. The combination of ALK-2 and ALK-3 also induced higher transcriptional activity than either receptor alone. Thus, ALK-2 and ALK-3 (or ALK-6) might synergistically induce osteoblast differentiation of C2C12 cells, possibly through efficient activation of downstream signaling pathways.