Bone morphogenetic protein (BMP)-3/osteogenin has a diverse spectrum of biological functions ranging from bone induction to developmental organogenesis. To clarify the role of BMP-3 during bone formation and maintenance, we investigated the expression of BMP-3 mRNA by in situ hybridization (ISH) on 4% paraformaldehyde fixed decalcified bone of normal and fractured bone of adult mice and in the whole body of the mouse fetus with digoxigenin-labeled single-stranded DNA probes generated by PCR. Our modified in situ hybridization technique at the electron microscopic level was also applied to identify specific cell types expressing BMP3 during endochondral ossification in the developing fetal bone. Besides its expression in the developing fetal bronchial epithelium and glial cells of the brain, BMP-3 expression was observed mainly on chondrocytic cells during maturation in the normal growth plate and the fractured callus of the adult long bone and the developing fetal woven bone, suggesting that BMP-3 expression was related not to differentiation of mesenchymal cells into the chondrocytic cell lineage but to the differentiation of immature chondrocytic cells into more mature chondrocytes.