Alexa Fluor 546 goat anti—mouse IgM (µ chain) is labeled with our bright, photostable, orange-fluorescent Alexa Fluor 546 dye and is prepared from antibodies purified by mouse IgM—affinity chromatography. The antibodies react specifically with IgM heavy chains (µ chains) and not with immunoglobulin light chains. To minimize crossreactivity, the goat anti—mouse IgM antibodies have been adsorbed human IgG_l, IgG_2a, IgG_2b, IgG₃, IgA, human serum and purified human paraproteins prior to labeling.