A series of fluorescent probes was used to analyze membrane lipid dynamics in promyelocytic leukemic cells sensitive (HL-60) or resistant (R-55) to phorbol diester induction of cell differentiation. When examined with the probe 1,6-diphenyl-1,3,5-hexatriene, which can penetrate the plasma membrane and intercalate in the lipids of both leaflets of the plasma membrane, as well as in organellar membranes, R-55 cells were found to have higher fluorescence anisotropy values, indicative of decreased lipid fluidity, as compared to HL-60 cells. In contrast, when HL-60 and R-55 cells were compared using a series of membrane-impermeant fluorophores (stachyose derivatives of anthroyloxystearate and pyrenebutyryl hydrazide) that incorporate only into the outer hemileaflet of the plasma membrane, no difference was observed in membrane lipid fluidity. Exposure to 12-O-tetradecanoylphorbol-13-acetate (10 ng/ml) for 24 hr decreased the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene in both HL-60 and R-55 cells, whereas by 48 hr only the HL-60 cells displayed the reduction. No effect on the fluorescence anisotropy of 1-(4'-trimethylammonium phenyl)-6-phenyl-1,3,5-hexatriene, which is believed to localize in the plasma membrane, was observed in R-55 cells exposed to 12-O-tetradecanoylphorbol-13-acetate (10 or 100 ng/ml), whereas HL-60 cells treated with 12-O-tetradecanoylphorbol-13-acetate (10 ng/ml) showed a marked reduction in the fluorescence anisotropy. These observations suggest that the ability of HL-60 cells to respond to 12-O-tetradecanoylphorbol-13-acetate may be affected by the physical state of the plasma membrane lipids and that the resistant phenotype is associated with decreased fluidity of either the inner leaflet of the plasma membrane and/or of the cytosolic organellar membranes.