There is a deficiency of complement receptor type 1 (CR1) on the erythrocytes of patients with systemic lupus erythematosus (SLE). This receptor is involved in the processing of immune complexes. Whether the deficiency is inherited or acquired has been the subject of controversy. A restriction fragment length polymorphism (RFLP), identified using a complementary DNA probe for CR1, has been correlated with the numeric expression of CR1 on normal erythrocytes. The gene frequency for the 2 alleles defined by this RFLP was compared in 44 patients with SLE (from 42 families), 43 of their consanguineous relatives, and 50 nonrelated normal subjects. The gene frequency for the alleles correlating with high and low expression of CR1 was 0.73 and 0.27, respectively, in the normal subjects. The gene frequency was not significantly different in the SLE patients. However, the SLE patients expressed fewer CR1 molecules per erythrocyte within each genotype, compared with normal subjects and compared with their consanguineous relatives. The low allele for numeric expression of CR1 on erythrocytes is not a disease susceptibility gene for SLE.