The horseradish peroxidase conjugate of F(ab')2 fragment of goat anti—mouse IgG 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.