The c-fos proto-oncogene is the cellular homologue of v-fos identified as the bone transforming gene of the FBJ and the FBR murine osteosarcoma viruses. We show here, using a sensitive in situ hybridization method, that the c-fos proto-oncogene is expressed in the cartilage, bone and tooth forming tissues during mouse development. This result suggests that the tumors observed after infection by the FBJ viral complex and c-fos overexpression in transgenic mice occur in those tissues in which c-fos is expressed during development.