Erythrocytes from 56 patients with systemic lupus erythematosus (SLE) were tested for the immune-adherence (C3b) receptor reactivity for incubation with aggregated human gamma-globulin (AHG) in the presence of complement. The reactivity of the C3b receptors was expressed as the highest two-fold dilution of AHG that induced haemagglutination. Erythrocytes from 37 (66%) of the SLE patients failed to show any detectable reactivity with AHG, whereas the erythrocytes of only 1 of 51 normal controls matched for age and sex were found to be unreactive. The defect of the C3b receptor reactivity was persistent and could not be restored even after SLE patients had gone into remission with steroid therapy. Moreover, the defect was found frequently in the relatives of patients without detectable immune-adherence reactivity. Owing to its high prevalence and persistence in SLE, the defective erythrocyte C3b receptor may be a useful marker for identifying SLE patients and those predisposed to the disease.