Mouse calvaria-derived osteoblastlike cells have been shown to produce macrophage colony-stimulating factor (M-CSF). This factor may be involved in osteoclastogenesis and thus in bone resorption. In the present study we investigated whether the production of M-CSF was altered in the osteopetrotic mouse mutant strain op/op, characterized by a decrease in osteoclast number and an impairment of bone resorption. Whole calvariae and cells, as well as skin and lung fibroblasts, of the op/op mouse were found to produce no measurable M-CSF, in contrast to tissue and cells derived from normal littermates. M-CSF was identified by colony assay in semisolid media and by inhibition of the biologic activity with antiserum against M-CSF. Furthermore, the number of resident macrophages, identified by F4/80 antigen (F4/80 Ag) immunohistochemistry, was drastically decreased in bone and bone marrow of the op/op mouse, but in skin these cells were normal in number and morphology. These findings suggest that both M-CSF and resident macrophages play a role in the mechanism of bone resorption. The op/op mouse appears to be a valuable model to further investigate such a hypothesis.