The membrane lipid fluidity of normal human erythrocytes was modified by enrichment and depletion in cholesterol, and the expression of I and SP1 antigens was assayed by quantitative hemagglutination from 4 degrees to 24 degrees C by use of a continuous flow system. Below 16 degrees--18 degrees C, cholesterol enrichment increased and cholesterol depletion decreased percent agglutination. As temperatures approached approximately 18 degrees--20 degrees C, differences in agglutination between modified and unmodified erythrocytes became insignificant despite marked differences in lipid fluidity at that temperature. Thus, fluidity changes alone cannot be responsible for the effect of membrane cholesterol on cold agglutination. In an additional study, the temperature dependence of a relative equilibrium association constant, estimated by probit analysis of percent agglutination at various antisera concentrations, was biphasic with a sharp break at 16 degrees C. Our studies are consistent with the hypothesis that I and Sp1 antigens preferentially partition into a lipid domain that forms during lateral phase separation of membrane lipid developing at low temperature. A resulting increase in antigen density would then become responsible for augmented agglutination by specific antibody.