The Alexa Fluor 647 donkey anti—goat IgG is labeled with superior far-red—fluorescent Alexa Fluor 647 dye and is prepared from affinity-purified antibodies that react with IgG heavy chains and all classes of immunoglobulin light chains from goat. To minimize crossreactivity, it has been adsorbed against rabbit, rat, mouse and human IgG. Fluorescence of this long-wavelength Alexa Fluor dye is not visible to the human eye but is readily detected by most imaging systems.