Ontogeny-related changes in megakaryocyte (MK) progenitor cells were a analyzed to further define the cellular hierarchy occurring during human MK development. CD34+ cells were selected from human low-density adult bone marrow (ABM) or unfractionated fetal bone marrow (FBM) and assayed for MK colony formation in a serum-depleted fibrin clot assay system. At days 3, 7, 12, 16, 21, and 28 of incubation, MK colonies were analyzed for colony number, size and number of foci of development. Unifocal CFU-MK-derived colonies cloned from FBM formed after fewer days of in vitro culture and were 2.6-fold larger than those colonies cloned from ABM, However, the frequency of CFU-MK derived colonies cloned from ABM was significantly greater. the MK colonies cloned from FBM morphologically consisted of both pure MK colonies and mixed MK-containing colonies, in which a core of CD41- cells were surrounded by CD41+MKs. Large colonies resembling the primitive burst-forming unit-MK (BFU-MK) also were assayable from both FBM and ABM. These BFU-MK-derived colonies appeared after fewer days of incubation when FBM was assayed compared to ABM, but at a significantly lower frequency. In addition, large unifocal MK colonies consisting of >300 cells (300-1000) appeared from cells cloned from FBM but not ABM. This type of colony, which appears to represent a unique type of MK progenitor cell, was termed a high proliferative potential cell-MK (HPPC-MK). such colonies represent a relatively rare MK progenitor. We conclude that there are a significant ontologic changes in human MK progenitor cell development. The kinetics of MK colony formation from ABM differs significantly from that of FBM. Furthermore, the proliferative capacity of fetal MK progenitor cells appear to be far greater than that of ABM. In addition, a more primitive lineage-restricted MK progenitor cell with extensive proliferative capacity, the HPPC-MK, was assayable exclusively from FBM.