The study was carried out in 121 patients with multiple sclerosis and a control group of 519 healthy individuals. In 13 cases of multiple sclerosis and 12 controls family studies were carried out. Complement receptors (C3bR) on the erythrocytes were demonstrated by the haemagglutination test using human IgG heat-aggregated and guinea pig serum which served as a source of complement. On the basis of haemagglutination intensity and results of radioimmunoassay three phenotypes of complement receptor were isolated: high HH(C3bRh/C3bRh) producing strong agglutination, medium HL (C3bRh/Cbl) producing weak agglutination, and low phenotype LL (C3bR1/Cbl) in which no visible agglutination of erythrocytes was noted. Considerable differences were observed in the distribution of these phenotypes between controls and multiple sclerosis patients in whom the low phenotype (C3bR1/C3bR1) was more frequent. Family studies of controls suggested presence of two codominant alleles C3bRh and C3bR1 with Mendelian inheritance while similar studies of multiple sclerosis patients it was found that e.g. a child with multiple sclerosis whose parents had high C3bR phenotypes, that it were homozygotes for the C3bRh gene, had low phenotype. On the other hand, the high phenotype was found in a child of a female multiple sclerosis patient with the low phenotype (C3bR1/C3bR1). These observations suggest that reduced expression of the complement receptor on the erythrocytes depends in multiple sclerosis on the disease process in the first place, and not on genetic factors.