The Src tyrosine kinase has been implicated in a wide variety of signal transduction pathways, yet despite the nearly ubiquitous expression of c-src, src−/− mice show only one major phenotype—osteopetrosis caused by an intrinsic defect in osteoclasts, the cells responsible for resorbing bone. To explore further the role of Src both in osteoclasts and other cell types, we have generated transgenic mice that express the wild-type and mutated versions of the chicken c-src proto-oncogene from the promoter of tartrate resistant acid phosphatase (TRAP), a gene that is expressed highly in osteoclasts. We demonstrate here that expression of a wild-type transgene in only a limited number of tissues can fully rescue the src−/− phenotype. Surprisingly, expression of kinase-defective alleles of c-srcalso reduces osteopetrosis in src−/− animals and partially rescues a defect in cytoskeletal organization observed in src−/− osteoclasts. These results suggest that there are essential kinase-independent functions for Src in vivo. Biochemical examination of osteoclasts from these mice suggest that Src may function in part by recruiting or activating other tyrosine kinases.