Hemopoietic populations offer by far the best available model systems for analyzing many aspects of the fundamental mechanisms controlling cell proliferation and differentiation. Semisolid cloning systems of high plating efficiency exist for normal and leukemic hemopoietic populations, and in these culture systems both proliferation and differentiation occur under defined culture conditions. Furthermore, the various differentiated end cells differ so extremely in morphology, membrane markers, and functional activity that a wide variety of parameters is available for monitoring differentiation. For the erythroid and granulocyte-macrophage systems, specific progenitor cells can be stimulated to generate large clones of differentiating progeny using the purified glycoprotein regulators erythropoietin, GM-CSF, and M-CSF. Exploitation of these systems will provide information on the events leading to proliferation and differentiation that should have general relevance for many other cell systems.