The cellular receptor for the C3b fragment of the third component of complement is a 205,000 molecular weight glycoprotein expressed by erythrocytes, polymorphonuclear leukocytes, monocytes, B lymphocytes, a subset of T lymphocytes and glomerular podocytes. The receptor molecule is a potent inhibitor of complement activation by both the alternative and classical pathways. It serves as a cofactor in the proteolytic degradation of C3b bound to immune complexes. On neutrophils and monocytes, the receptor enhances immunoglobulin-dependent phagocytosis of opsonized particles and triggers internalization of soluble ligands bearing C3b. The number of C3b receptor molecules expressed on erythrocytes is genetically determined and was found to be low in patients with systemic lupus erythematosus: these abnormalities when associated with a low number of receptors in the kidney of patients with non-systemic lupus erythematosus nephritis may predispose to immune complex diseases.