Osteoclasts are bone resorbing cells of hematopoietic origin; however, a progenitor cell population that gives rise to mature osteoclasts remains elusive. We have characterized a unique cell surface phenotype of clonogenic osteoclast progenitors (colony-forming unit-osteoclast [CFU-O]) and obtained a marrow cell population selectively enriched for these progenitors. Whole bone marrow cells were sequentially separated based on physical and cell surface characteristics, and the presence of CFU-O and other hematopoietic progenitors was examined. CFU-O was enriched in a nonadherent, low-density, lineage-marker-negative (Lin-), Thy1.2-negative (Thy1.2-), Sca1-negative (Sca1-), and c-kit-positive (c-kit+) population, as were the progenitors that were responsive to macrophage-colony-stimulating factor(CSF; CFU-M), granulocyte-macrophage-CSF (CFU-GM), and stem cell factor (CFU-SCF). When the Lin-Thy1.2-Sca1- population was divided into c-kithigh and c-kitlow populations based on c-kit fluorescence, over 88% of CFU-M, CFU-GM, and CFU-SCF were found in the c-kithigh population. In relation to the above mentioned hematopoietic progenitors, CFU-O was significantly higher in the c-kitlow population: 80% of progenitors present in the c-kitlow population were CFU-O. The CFU-O in both c-kithigh and c-kitlow populations showed key features of the osteoclast: multinucleated tartrate-resistant acid phosphatase-positive cell formation, expressions of vitronectin receptors, c-src and calcitonin receptors, and bone resorption. We have identified a progenitor cell population in the earliest stage of the osteoclast lineage so far described and developed a method to isolate it from other hematopoietic progenitors. This should help pave the way to understand the molecular mechanisms of osteoclast differentiation.