The murine OP-1 gene (EMBL accession No. X56906) encoding the homolog of human osteogenic protein-1 was isolated from cDNA and genomic libraries using human OP-1 cDNA as probe. The deduced murine OP-1 amino acid sequence revealed 11 amino acids changes, three of them in the mature protein. Murine OP-1 probes were used for analysis of OP-1 mRNA in mouse embryo and organ tissues. Northern blot hybridization revealed multiple mRNA species: the major species of 2.2 kb, minor species of 1.8 and 2.4 kb and a large 4 kb species, which may represent alternative splices. Tissue specific expression was studied in brain, lung, heart, liver, spleen, kidney, adrenal and bladder tissue. Maximal levels of OP-1 mRNA were found in kidney which may explain the phenomenon of epithelial osteogenesis, first described by Huggins in 1931 using epithelium from the urinary tract. Moreover, our data suggest that kidneys may be the main site of OP-1 synthesis, even though it is distant from its physiological site of action, skeletal bone.