In acute leukemia a neoplastic cell population escapes normal maturation impulses and expands selectively at the expense of normal hematopoiesis. How human acute leukemia cells react in the interplay of regulatory factors that control hematopoiesis and whether growth-factor abnormalities determine abnormal growth is only beginning to become evident. In this chapter developments resulting from the cloning of hematopoietic growth factors and the characterization of hematopoietic progenitor cells have been discussed.