Treatment of non-T/non-B human leukemic cell line REH with 5 X 10(-9) M 12-O-tetradecanoylphorbol-13-acetate (TPA) at 37 degrees resulted in their adherence to culture flasks by 24 to 36 hr and, after 72 hr, the entire surface of the flask/coverslips became covered with macrophage-like cells containing pseudopodia. Wright-Giemsa-stained untreated cells had blast morphology, whereas TPA-treated cells (adherent or excess cells remaining in suspension) had characteristic morphology of macrophages and phagocytized large numbers of latex beads. Untreated REH cells were negative for nitroblue tetrazolium reduction, Sudan Black B, and peroxidase, and they were weakly positive for periodic acid-Schiff, acid phosphatase, chloroacetate esterase (pH 6.8), and nonspecific (naphthol AS-D acetate, pH 6.8) esterase, whereas TPA-treated cells (adherent or in suspension) gave strong reaction for these stains except for peroxidase and chloroacetate esterase which showed moderate reaction. Furthermore, the nonspecific esterase activity of TPA-treated cells and weak activity in 10% of untreated cells was strongly inhibited by NaF, a characteristic of monocytic series of cells. Lysozyme activity was not detected in culture supernatant from control or TPA-treated cells. No cytoplasmic immunoglobulin was detected in untreated or TPA-treated cells, and the monocyte/granulocyte antigen (detected by MCS-2 monoclonal antibody) which was absent from untreated REH cells was expressed in TPA-treated cells. TPA-treated cells lost common acute lymphoblastic leukemia antigen but showed significantly elevated expression of histocompatibility locus DR antigen. Terminal transferase estimated by immunofluorescence and biochemical assay was high in untreated REH cells, whereas TPA-treated cells were negative in terminal transferase immunofluorescence and had only negligible terminal transferase activity in biochemical assay. All these changes in REH cells observed on TPA treatment represent the differentiation of a human leukemic non-T/non-B-cell line to macrophage-like cells for the first time which indicates that some non-T/non-B acute lymphoblastic leukemia cells may have latent monocyte-like phenotype.