Alexa Fluor 555 goat anti—mouse IgM (µ chain) is labeled with our exceptionally bright, orange-fluorescent Alexa Fluor 555 dye (which is spectrally similar to Cy3) 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 against human IgG_l, IgG_2a, IgG_2b, IgG₃, IgA, human serum and purified human paraproteins prior to labeling.