C3b receptor (CR1) on erythrocytes from 23 patients with systemic lupus erythematosus (SLE) and 124 normal controls was determined by immune adherence hemagglutination (IAHA) and radioimmunoassay. The binding of radiolabeled monoclonal anti-CR1 to erythrocytes and their lysate was distributed continuously in a wide range. The majority of SLE patients showed low binding by both assays. CR1 sites on erythrocytes were determined also by Scatchard plot analysis and standardized by the number of similarly determined lectin-binding sites that served as a measure of erythrocyte surface. The numbers of standardized CR1 sites were classified as high, intermediate, and low. Thirty-six percent of control subjects had high numbers of CR1 sites, 53% had intermediate numbers, and 11% had low numbers. Of SLE patients, the numbers of CR1 sites were high in 0%, medium in 52%, and low in 48%. Negative IAHA was found in 10 controls (8%), all of whom had low numbers of standardized CR1 sites. Among 13 SLE patients with negative IAHA, 11 had low numbers of CR1 sites and the remaining 2 had low intermediate numbers. IAHA, therefore, was particularly efficient in detecting the low numbers of CR1 sites in SLE, which would impair the disposal of circulating immune complexes and accelerate the development of tissue injuries.