The erythrocytes of patients with systemic lupus erythematosus have been shown to have a decreased number of receptors for the major cleavage fragment of the third component of complement (C3b). We studied the basis for this decreased number of C3b receptors by measuring the uptake of anti-C3b-receptor antibody on cells from 113 normal subjects, 38 patients with systemic lupus erythematosus, 14 of their spouses, and 47 of their relatives. The normal subjects had 5014 +/- (mean +/- S.E.M.) receptor sites per cell, but the patients and their relatives had significantly fewer sites--2809 +/- 241 and 3167 +/- 196, respectively (P less than 0.0005). The number of sites in the patients' spouses did not differ from normal (P greater than 0.3). Three phenotypes, indicated by the numbers of receptors, occurred in the normal population; a high phenotype (HH, with 5500 to 8500 sites per cell), an intermediate phenotype (HL, with 3000 to 5499), and a low phenotype (LL, less than 3000). These three phenotypes were present in 34, 54, and 12 percent, respectively, of the normal subjects; in contrast, 5, 42, and 53 per cent of patients had these respective phenotypes. Pedigree analyses indicated that these phenotypes were inherited in an autosomal codominant manner. We conclude that the decreased number of C3b receptors in lupus is inherited, not acquired.