The Thy-l. l “Sea-1 h’Lin-‘l” population, representing 0.05% of C57BL/Ka-Thy-l. 1 bone marrow, is highly enriched for hematopoietic stem cells and includes all multipotent progenitors in this mouse strain; however, the functional reconstituting activity of this fraction is heterogeneous. Only around 25% of clonal reconstltutions by cells from this population are long term; remaining clones yield transient multilineage reconstitutlons. By fractionating based on lineage marker expression, the Thy-l. l’OSca-lhlLitP population has been resolved into three subpopulations: Lin-Mac-1-CD4-; Lin-Mac-l “CD4-; and Mac-l ‘OCD41. of these, only the Lin-Mac-l-CD4-population is highly enriched for long-term reconstituting hematopoietlc stem cells. A comparison of transient and long-term multipotent progenitors indicates that long-term progenitors have less CFU-S activity, are equally radioprotective, and are less frequently in cell cycle. The ability to predict the longevity of reconstitution based on lineage marker expression indicates that reconstitution potential is deterministic, not stochastic.