The anti-IgM antibody binds to circulating IgM as well as to IgM antibodies bound to the FcμR. The FcμR is a transmembrane single sialoglycoprotein of 60 kD with O-linked oligosaccharides. It contains an extracellular Ig-like domain homologous to two other IgM-binding receptors (polymeric Ig receptor and Fcα/μR) but exhibits an exclusive Fcμ-binding specificity. Unlike other FcRs, the major cell types expressing FcμR are adaptive immune cells, including B and T lymphocytes. The FcμR 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 μ- pre-B cells and isotype-switched B cells may express the FcμR. The FcμR 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.