The tumor-promoting ester 4 beta-phorbol 12-myristate 13-acetate (PMA) has been shown to induce the differentiation of the immature monocytelike cell line U-937 c in vitro into a heterogeneous population of cells, including small" dense" cells, large vacuolized or" foamy" cells, spindle-shaped cells, and cells with multiple filopodia (" stellate" cells). The effect of PMA was dose-and time-dependent, the optimal conditions being 40-162 nM PMA for 48 hours. The minimum time of exposure to PMA to ensure further differentiation of U-937 cells was about 5 hours. The PMA-stimulated cells acquired morphologic, ultrastructural, and functional characteristics typical of cells of the monocyte/macrophage lineage. The PMA-treated U-937 cells became adherent, ceased to proliferate, and exhibited increased expression of monocyte-specific antigens (Leu-M2,-M3, HLADr), surface receptors (FcR, C3bR), enzymes (nonspecific esterase, transglutaminase), and ability to mediate chemotaxis, phagocytosis, superoxide anion production, and antibody-dependent cytotoxicity reactions. The induced cells lost their morphologic differentiation and ability to attach to surfaces and regained proliferative capacity upon repeated subculture in PMA-free media.