The reciprocal transplantation of hematopoietic tissues was carried out on young, lethally irradiated mice of inbred, microphthalmic stock. The cell infusions prepared from the bone marrow or spleen of a normal littermate fully restored capacity to resorb bone and cartilage in the osteopetrotic recipients. Conversely, cell infusions prepared from the spleen of microphthalmic mice induced osteopetrosis in their irradiated, normal littermates. It is concluded that resorption of skeletal matrix is controlled by migratory cells, possibly osteoclastic progenitors, derived from the myelogenous tissues. No evidence was obtained to suggest that skeletal changes observed in the experimental animals were mediated by a graft-vs.-host reaction. The earliest skeletal changes in the experimental mice were detected 2 wk after onset which may represent the length or time required to replace the osteoclast population of the mouse.