Malignant osteopetrosis is a disorder characterized by a deficiency in osteoclast number or function. In one animal model of osteopetrosis, the op/op mouse, macrophage colony-stimulating factor (M-CSF) is absent, and the administration of M-CSF corrects the defects. We evaluated the serum of 13 patients with malignant osteopetrosis by an M-CSF radioimmunoassay to determine if a quantitative M-CSF deficiency existed in these patients. All patients had M-CSF present in levels equal to or higher than control serum. In addition, serum from 6 osteopetrotic patients was tested in a bioassay to determine if the M-CSF present is biologically active, and in all cases there was demonstrable activity in these samples. We provide evidence that deficiency of circulating M-CSF is unlikely to be a major contributor to the etiologic basis for the majority of children with malignant osteopetrosis.