The binding of complement (C)‐solubilized125I bovine serum albumin (BSA) anti‐BSA immune complexes (IC) to CR1 receptors on human red blond cells (RBC‐CR1) was studied. The binding of IC to CRI was strongly dependent on the molar antigen lo antibody ratio, and IC formed in moderate antigen excess showed no binding. IC solubilized, in 50% human serum in the presence of autologous RBC bound rapidly lo RBC‐CRI, with maximal binding within less than 1 min at 37°C. Release of CRI‐hound IC under these conditions occurred slowly, requiring more than.30 min. Only binding of ‘partially’ solubilized, e.g., anti C3c (C4c) and conglutinin‐reactive 1C occurred, whereas fully solubilized complexes (IC‐C3dg. C4d) showed virtually no binding. Solubilization of IC in the presence of Mg‐EGTA or in C2‐deficient serum resulted in a markedly delayed binding of IC ti RBC, indicating the importance of an intact classical pathway in preparing the IC for binding to RBC‐CR1. C‐solobilized IC could be absorbed to solid‐phase conglutinin or antibody to C3c abd C4c, and tgese kugabds were able to inhibit the binding of solubilized IC to RBC. Heparin also exerted a marked, dose‐dependent inhibitory effect on the binding of presolubilized IC to RBC‐CR1, whereas the binding was unaffected by the addition of monosaccharides or by the