Alexa Fluor 647 F(ab')2 fragment of goat anti—rabbit IgG 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 that have been adsorbed against pooled human serum, mouse serum, mouse plasmacytoma/hybridoma proteins and purified human paraproteins 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.