Rh-positive erythrocytes were enriched and depleted of membrane cholesterol, and the mediated change in the degree of exposure of the D antigens was determined by fluorescence-activated cell sorting, using indirect fluorescent antibody labeling. The results are compatible with a model in which the expression of the D antigens can be modulated significantly by the lipid microviscosity (eta). At a high cholesterol-to-phospholipid ratio (C/PL) of 1.55, which corresponds to eta (25 degrees C) = 7.5 poise (1 poise = 0.1 Pa.sec), the relative detectable number of D antigens was about double than that at C/PL = 0.65, eta (25 degrees C) = 4.1 poise. In analogous experiments similar fluidity changes resulted in only about 20% modulation of expression of the A1 antigen, suggesting that in the native state this antigen is already well exposed on the erythrocyte surface. This type of antigenic modulation may also operate in vivo, and may thus bear some fundamental implications on tumor immunology and autoimmune diseases.