Streptavidin binds to biotin with high affinity. Streptavidin-HRP is useful for detecting biotinylated antibodies, e.g., in ELISA, ELISPOT, IHC or Western blotting. For ELISA applications, HRP will act on soluble substrates, such as ABTS or TMB, to yield a colorimetric reaction. For ELISPOT or Western blotting applications, HRP will act on precipitating substrates, such as 4CN, to yield a colorimetric reaction.