Methodology has been developed that enables virtually complete purification and abundant recovery of early hematopoietic progenitors from normal human adult peripheral blood. A fraction of the pure progenitors is multipotent (generates mixed colonies) and exhibits self-renewal capacity (gives rise to blast cell colonies). This methodology provides a fundamental tool for basic and clinical studies on hematopoiesis. Optimal in vitro cloning of virtually pure progenitors requires not only the stimulatory effect of interleukin-3, granulocyte-macrophage colony-stimulating factor, and erythropoietin, but also the permissive action of basic fibroblast growth factor. These findings suggest a regulatory role for this growth factor in early hematopoiesis.