The complement receptor for C3b of the epithelial cells of human glomeruli is structurally and functionally very similar or identical to CR1, the complement receptor for C3b and C4b present on the membrane of red cells and leukocytes. Four monoclonal antibodies directed against separate epitopes of CR1 react with an antigen in the glomeruli, which appears to be present on the epithelial podocytes. Moreover, the monoclonal antibodies very effectively inhibit the binding of C3b-bearing red cells to the glomeruli. The pattern of immunofluorescence of the receptor was normal or slightly altered in patients with minimal change disease, mesangial proliferative glomerulonephritis (GN), or idiopathic membranous GN. Glomeruli with endocapillary proliferation showed some attenuation of staining. Glomeruli in which the capillary tuft architecture was altered, or of patients with systemic lupus erythematosus, or of patients with diffuse diabetic nephropathy tended to have few foci or no staining for the receptor. No correlation was found between the intensities of staining of the C3b receptor and of C3 antigen deposited in the glomeruli.