Lactoperoxidase-catalyzed cell surface radioiodination was employed to radiolabel membrane polypeptides of a murine macrophage cell line P388D1. Optimal conditions for radioiodination of P388D1 cells were determined and were found to differ from conditions used to label lymphoid cells. SDS-polyacrylamide gel electrophoresis of detergent soluble membrane polypeptides revealed that 9 to 10 molecular species from 1 × 105 to 0.15 × 105 daltons were labeled. Radioiodinated, Triton X-100 extracted P388D1 membrane polypeptides were subjected to affinity chromatography on aggregated IgG:Sepharose columns. Elution of the bound polypeptides and analysis by SDS-polyacrylamide gel electrophoresis revealed polypeptides with an apparent molecular size of 8 × 104, which possess binding affinity for the Fc portion of aggregated IgG. The 8 × 104 dalton membrane polypeptides do not readily aggregate, are resistant to degradation, are not composed of disulfide-linked subunits, and do not appear to contain much carbohydrate. Cellular binding characteristics paraleled the binding of soluble receptor for sieved fractions of aggregated IgG suggesting that these polypeptides may be responsible for the in situ binding of aggregated IgG to P388D1 cells.