The regulation of Mac-1, LFA-1, and p150,95 expression during leukocyte differentiation was examined. LFA-1 was present on almost all cell types studied. Both Mac-1 and p150,95 were present on the more mature cells of the myelomonocytic series, but only p150,95 was detected on some B cell lines and cloned cytotoxic T lymphocytes. Phorbol myristate acetate (PMA) stimulation of B chronic lymphocytic leukemia cells dramatically increased p150,95 expression. The resultant Mac-1, LFA-1, p150,95 phenotype resembled hairy cell leukemia, a B cell plasmacytoid leukemia. The promonocytic cell line U937 and the promyeloblastic cell line HL-60 expressed only LFA-1. Monocytic differentiation of U937 cells was stimulated by PMA, and induced the concomitant expression of Mac-1 and p150,95, with more p150,95 induced than Mac-1. Granulocyte/macrophage colony-stimulating factor (GM-CSF) stimulation of U937 cells gave similar results. PMA-stimulated monocytic differentiation of the HL-60 cell line also induced expression of both Mac-1 and p150,95. The number of p150,95 molecules on PMA-stimulated U937 and HL-60 cells were 5 X 10(5) and 3 X 10(5), respectively. Retinoic acid stimulated myeloid differentiation of HL-60 cells and induced expression of both Mac-1 and p150,95. These cells acquired a Mac-1, LFA-1, p150,95 profile that resembled that of granulocytes, with more Mac-1 than p150,95 induced. GM-CSF stimulation of HL-60 cells induced a similar Mac-1 and p150,95 phenotype. The contributions of Mac-1, LFA-1, and p150,95 to aggregation of PMA-differentiated U937 cells were assessed. Monoclonal antibodies to the beta subunit and the LFA-1 alpha subunit, but not those to p150,95 or Mac-1 alpha subunit, inhibited this homotypic adherence.