The osteoclast is known to be derived from a marrow-residing precursor that is a member of the mononuclear phagocyte family, but the means by which this cell moves from marrow to bone is unknown. We herein demonstrate that mononuclear progenitors capable of differentiating, in vitro, into cells exhibiting the osteoclast phenotype circulate in chickens. The mononuclear fraction was isolated on a density gradient from blood drawn from calcium-deprived laying hens and the plastic-adherent population was obtained. These cells are members of the mononuclear phagocyte family, as demonstrated by nonspecific esterase and tartrate-resistant acid phosphatase (TRAP) activities, expression of the macrophage-specific mannose receptor, and their ability to phagocytose latex particles. When cultured in the presence of devitalized bone, these cells undergo progressive multinucleation and ultimately become essentially indistinguishable from isolated osteoclasts and those generated from bone marrow precursors. Specifically, the blood-derived polykaryons are TRAP-positive, exhibit characteristic ruffled membranes, and express the osteoclast antigens 121F and 23C6. When placed on bone slices, these cells from typical resorptive "pits." Moreover, when cultured with 3H-proline-labeled bone, the blood monocytegenerated osteoclasts mobilize matrix as effectively as those derived from marrow. Thus, osteoclast precursors circulate in the blood of laying hens and can be induced to differentiate in vitro.