Acute myeloblastic leukemia, like certain other hematologic disorders, originates in pluripotent stem cells. Two general biologic processes underlie development of the disease. Over long times, clonal progression leads from normal polyclonal hemopoiesis through clonal preleukemia to leukemia. Overt leukemia is characterized by the emergence of blast cell populations. Over shorter times, clonal expansion yields cellular diversity based upon randomizing events. The analysis indicates that that blast population is of crucial importance. Characteristics of a colony assay for blast cell progenitors are presented.