Osteopetrotic (op/op) mice have a severe deficiency of osteoclasts, monocytes, and peritoneal macrophages because of a defect in the production of functional macrophage colony-stimulating factor (M-CSF) resulting from a mutation within the M-CSF gene. In this study, we examined whether daily 5-microgram injections of purified recombinant human M-CSF (rhM-CSF) for 14 d would cure these deficiencies in the mutant mice. Monocytes in the peripheral blood of the op/op mice were significantly increased in number after subcutaneous injections of the factor two or three times a day. In contrast, osteopetrosis in the long bones of op/op mice was completely cured by only one injection of rhM-CSF per day. Bone trabeculae in the diaphyses were removed. Many osteoclasts were detected on the surface of bone trabeculae in the metaphyses. Although development of tooth germs of uninjected op/op mice was impaired, rhM-CSF injection restored the development of molar tooth germs and led to tooth eruption as a consequence of the recovery of bone-resorbing activity. These results demonstrate that M-CSF is one of the factors responsible for the differentiation of osteoclasts and monocyte/macrophages under physiological conditions.