CR1 exhibits a molecular weight polymorphism and variability in the number of C3b-binding sites. Because this may affect immune complex clearance, we used erythrocytes to investigate the CR1 size polymorphism in SLE patients from three ethnic groups. The CR1-C allele was found more frequently in African-Americans, but the frequency did not differ between controls (10%, n = 63) and SLE patients (9%, n = 79). A 160-kD band similar to CR1-C was noted in a number of patients and was shown to be a proteolytic cleavage fragment. The study shows that the smallest form of CR1, i.e. CR1-C, is not a genetic risk factor for SLE and that the frequencies of the CR1 structural alleles do not differ from race-matched healthy controls.