Alexa Fluor 633 goat anti—mouse IgM (µ chain) is labeled with our far-red—fluorescent Alexa Fluor 633 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.