The direct membrane effect of propranolol was studied in vitro on washed platelet preparations. Propranolol has been reported to inhibit platelet aggregation by mechanisms unrelated to its beta-blocking activity. In the present study, the drug was found to enhance 1-anilino-8-naphthalene sulfonate binding to platelet membrane by increasing the number of binding sites. Steady-state anisotropy was studied by labeling the platelets with the hydrophobic fluorescent probe 1,6-diphenyl-1,3,5-hexatriene. Propranolol was observed to decrease the equivalent microviscosity of the membrane. When the infinitely slow decaying component of fluorescence anisotropy (r infinity), which is proportional to the square of lipid order parameter, was calculated from the anisotropy data, a decrease in these parameters was also indicated. A higher fusion activation energy for viscosity in the propranolol-treated platelets reflected a lesser degree of order of the hydrocarbon chains in the lipid bilayer.