IL-2 promotes the growth and differentiation of immature myeloid cells. FACS-isolated Mac-1^loIL-2Rβ⁺bone marrow cells (before culture) were cultured for 7 days in complete media in the absence (media) or presence (+IL-2) of 100 U/mL of IL-2 after which the cells were harvested, counted, and cytocentrifuge preparations made. Upper panel: addition of IL-2 resulted in significant increase in the number of cells recovered at the end of culture (P < .001). Lower panel: cytocentrifuge preparations of Mac-1^loIL-2Rβ⁺ cells before (before culture) and 7 days after culture in the absence (media) or presence (+IL-2) of IL-2. In the presence of IL-2, Mac-1^loIL-2Rβ⁺ cells give rise to macrophages as well as mature granulocytes. Magnification 1,000X. The results are representative of three independent experiments.