Two monoclonal antibodies, designated MRC OX-41 and MRC OX-42, have been shown to label subsets of macrophages. Using immunoperoxidase and immunofluorescence analysis, tissue macrophages were shown to be heterogeneous with respect to binding of MRC OX-41 and MRC OX-42 antibodies. Although both antibodies labelled subsets of macrophages, the antibodies also reacted with granulocytes and dendritic cells. The antigens recognized by these antibodies were identified by metabolic and cell surface labelling followed by immunoprecipitation and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). MRC OX-41 recognized a surface protein of 110,000-120,000 MW, while MRC OX-42 immunoprecipitated three polypeptides with molecular weights of 160,000, 103,000 and 95,000. The Fab fragment of MRC OX-42 antibody inhibited complement-mediated rosette formation between sensitized erythrocytes and rat macrophages and granulocytes. Membrane molecules with similar biochemical and functional properties to MRC OX-42 antigen have been identified in mouse and man as the receptors for iC3b, and it is probable that MRC OX-42 antibody recognizes the rat homologue of the receptors in these other species.