Colony forming cells (CFC) with high proliferative potential have been detected in nutrient agar cultures of human bone marrow cells containing recombinant human interleukin-3 (IL-3) and granulocyte macrophage colony stimulating factor (GM-CSF). These CFC were detected by the formation of large colonies with diameters greater than 0.5 mm and containing approximately 50,000 cells after 28 days incubation. The incidence of these CFC was only two in 100,000 normal bone marrow cells; however, bone marrow from patients treated with 5-fluorouracil contained up to sevenfold higher numbers of these CFC. The characteristics of these CFC, multifactor-responsive progenitors with high proliferative potential, requiring a prolonged growth period in culture and showing a relative preservation in marrow from individuals pretreated with 5-fluorouracil, are consistent with a human cell type equivalent to the primitive murine progenitor termed HPP-CFC.