Alexa Fluor 647 goat anti—mouse IgM (µ chain) is labeled with our exceptionally bright, far-red—fluorescent Alexa Fluor 647 dye (which is spectrally similar to Cy5 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. Fluorescence of this long-wavelength Alexa Fluor dye is not visible to the human eye but is readily detected by most imaging systems.