The biotin-XX conjugate of F(ab')2 fragment of goat anti—rabbit IgG is prepared from antibodies that have been adsorbed against human IgG, human serum, mouse IgG and bovine 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.