Abstract Human bone marrow cells have been fractionated by velocity sedimentation at unit gravity. Fractions were analyzed for cell morphology, number of nucleated cells and myeloid colony-forming cells (CFU-C's). Colony formation was assayed with the following CSF preparations: serum-free HLCM and two electrophoretically distinct CSF fractions (CSF-A and CSF-B) purified from the same source. Two distinct CFU-C populations were found. One, a rapidly sedimenting (7.2 to 8.0 mm/hr), population exhibited colonies after 7 days of culture in response to HLCM and CSF-A only. The second, a more slowly sedimenting (6.5 mm/hr) CFU-C peak, did not exhibit colonies until 11 days of culture and did so in response to all three CSF's tested. The results indicate that human bone marrow CFU-C's are heterogeneous and that the two purified CSF fractions from human lung have different CFU-C specificity.