Fluorescein isothiocyanate (FITC)-conjugated goat anti-mouse IgM, µ chain specific was isolated from antisera and reacts with the heavy chain region of mouse IgM but not with IgG or the light chains of mouse immunoglobulins. This antibody has minimal cross-reactivity to normal mouse IgG, human, cow, and horse serum proteins, but may react with IgM from other species. No antibody was detected against non-immunoglobulin serum proteins.