The op/op variant of murine osteopetrosis is a recessive mutation characterized by impaired bone resorption due to lack of osteoclasts. Cultured osteoblasts and fibroblasts from this mutant do not secrete M-CSF activity and resident macrophages are absent in bone marrow. This failure has been related to a mutation within the M-CSF coding region. We report now that the administration of recombinant human M-CSF (rhM-CSF) corrects in vivo the impaired bone resorption in this animal. The treatment restores the bone marrow cavity virtually absent in the op/op animal and induces the appearance of resorbing osteoclasts and of resident bone marrow macrophages. This proves that the deficiency of M-CSF is the cause of the op/op bone disorder and that this cytokine is directly or indirectly necessary for physiological osteoclastogenesis, the resulting bone resorption and for the establishment of bone marrow hemopoiesis.