Examination of the op/op mouse disclosed marked reduction and abnormal differentiation of osteoclasts in the bones and of tissue-specific macrophages in various visceral organs and tissues. Most of these macrophages were immature as judged by ultrastructural criteria. In co-cultures of normal mouse bone marrow cells with fibroblast cell lines prepared from the lungs of the op/op mice, a defective differentiation of monocytes into macrophages was confirmed, supporting previous evidence that the fibroblast cell lines of the mutant mouse failed to produce functional macrophage colony-stimulating factor (M-CSF/CSF-1). In such co-cultures, however, a small number of macrophages apparently mature under the influence of granulocyte/macrophage colony-stimulating factor (GM-CSF) produced by the op/op fibroblast cell lines. In the mutant mice, the numbers of macrophages in the uterine wall and ovaries were severely reduced. Compared with the tissues of normal littermates, those of the mutants contained about 60% fewer macrophages in many tissues. This suggests that an M-CSF-independent population of macrophages is derived from granulocyte/macrophage-colony-forming cells (GM-CFC) or earlier hematopoietic progenitors.