Osteopetrosis is a congenital disease in which the bone marrow is replaced by osteoid, resulting in secondary extrameduUary hematopoiesis. In 3 patients with osteopetrosis, circulating myeloid and erythroid progenitors were assayed. Erythroid burst-forming units were increased above normal by a factor of five and the myeloid colony-forming units were increased by a factor of ten. Myeloid cluster-forming units and erythroid colony-forming units, the more differentiated progenitor cells normally found only in the bone marrow, could also be detected in the peripheral blood of the 3 patients. The appearance of the differentiated progenitors in the peripheral blood suggests that extrameduUary hematopoietic sites, in contrast to the bone marrow, may release progenitor cells more readily into the circulation. If this is the case, culturing progenitor cells may provide a sensitive noninvasive laboratory test to determine the presence of extrameduUary hematopoiesis.