The Alexa Fluor 633 F(ab')2 fragment of goat anti—mouse IgG is labeled with our superior far-red—fluorescent Alexa Fluor 633 dye and is prepared from antibodies that have been adsorbed against human IgG and serum to minimize crossreactivity. Conjugates of F(ab')2 fragments are sometimes preferable to whole antibody conjugates for secondary detection, since the absence of the Fc region in F(ab')2 fragments prevents interactions with Fc receptor-bearing membranes. Fluorescence of this long-wavelength Alexa Fluor dye is not visible to the human eye but is readily detected by most imaging systems.