Complement receptor type 1 (C3b/C4b receptor, CR1) is known to be present in human glomerular epithelial cells (GEC) in vivo. The presence of CR1 has not been documented in rat glomeruli, although cultured rat GEC appear to express CR1 based upon their ability to rosette with complement-coated erythrocytes. In this study, we establish that CR1 is present in cultured rat GEC: (1) by isolating a 200 kDa protein from detergent-solubilized cultured rat GEC through the use of C3b affinity chromatography; (2) by Western blotting studies demonstrating reactivity of anti-human CR1 antibodies with this protein from cultured GEC; and (3) by demonstrating that C3b binding to GEC monolayers exhibits low affinity and that an estimate of the number of binding sites is 6700 per cell, both of which are comparable to that seen for CR1 in human blood cells. Furthermore, we show that CR1 is also present in rat glomeruli by Western blotting studies with anti-human CR1. Anti-human CR1 also identifies a 70 kDa protein from cultured GEC and isolated glomeruli. This 70 kDa protein is likely to be the CR1-like protein, designated Crry, which was initially identified in the mouse and has significant homology to human CR1. Crry may be present in rat GEC instead of decay accelerating factor, which is present in human GEC.