The study was carried out in patients with MS (n-121) and in control group (n-519). On the basis of haemagglutination intensity and results of radioimmunoassay three phenotypes of complement receptor were classified: high HH, medium HL, and low LL. Considerable differences were observed in the distribution of these phenotypes between controls and MS in whom the low phenotype was more frequent. Family studies suggest that reduced expression of the complement receptor on the erythrocytes depends in MS on the disease process in the first place, and not on genetic factors.