We have identified a distinct population of colony-forming cells that give rise to mononuclear cells expressing an enzyme marker and other features of the osteoclast in bone marrow cultures stimulated by conditioned medium of a murine tumor cell line. These colony-forming cells were defined as osteoclast colony-forming units (CFU-O). The tumor cell-derived activity was recently isolated and was named osteoclast colony-stimulating factor (O-CSF). To understand the development of osteoclast progenitors and to clarify the relationship of osteoclast progenitors to other hematopoietic progenitors, we examined CFU-O in hematopoietic tissues obtained from normal adult mice, mouse fetuses, and mice with 5-fluorouracil (5FU) treatment. CFU-O were present in the adult mouse bone marrow, adherent cell-depleted marrow, in the spleen, and in the day 14 fetal liver, with an incidence similar to other hematopoietic progenitors. The culture period required for the development of CFU-O-derived colonies in vitro and the manner in which CFU-O responded to 5FU suggested that CFU-O belonged to a relatively primitive progenitor population; they are clearly more immature than macrophage progenitors that respond to macrophage-CSF, but more mature than multilineage progenitors that respond to stem cell factor. Our studies have defined and characterized an osteoclast progenitor and distinguished it from other hematopoietic progenitors for the first time.