We studied membrane fluidity in living polymorphonuclear leukocytes and in erythrocytes of 8 psoriatic patients, using fluorescence polarization of 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene. In erythrocyte membranes of psoriatic patients we observed a decrease of fluidity, these changes were not associated to relevant modifications of the cholesterol to phospholipid molar ratio. Moreover, we observed a decrease in polymorphonuclear leukocytes membrane fluidity. Our results indicate changes in membrane fluidity involving membranes different from the epidermal cells and suggest the hypothesis of a defect in membrane-cytoskeleton interactions in psoriasis.