An IgM-binding protein of approximately 60 kD has been identified on activated B cells, but not on resting and activated T cells, monocytes, or granulocytes. Here, we characterize this IgM-binding protein as a receptor for the Fc portion (CH3 and/or CH4 domains) of IgM molecules (Fc microR). The Fc microR can be expressed as a cell surface activation antigen throughout the pre-B and B cell stages in differentiation. Receptor expression is not directly linked with IgM production, as both mu- pre-B cells and isotype-switched B cells may express the Fc microR. The receptor molecules produced by both pre-B and B cells are identical in size and are characterized as an acidic sialoglycoprotein with O-linked, but no N-linked, oligosaccharide. The Fc microR is anchored to the surface of B-lineage cells via a glycosyl phosphatidylinositol linkage. The Fc microR is thus the third member of a family of Fc receptors expressed on B-lineage cells, and its preferential expression on activated B cells suggests a potential role in the response to antigens.