Horseradish peroxidase (HRP) conjugates are prepared by a modified Nakane and Kawaoi procedure (J. Histochem. Cytochem. 1974. 22, 1084). Peroxidase conjugates are commonly used for immunohistochemistry, Western blotting, and ELISA. Affinity-purified anti-horseradish peroxidase and conjugates are available for detection of horseradish peroxidase antigen or for signal amplification of HRP-containing reagents.