Horse antibody raised against affinity purified mouse and rabbit IgGs was purified and optimally labeled withBiotin. The cross-reactivities that are likely to interfere with specific labeling are removed by solid phase adsorption chromatography. TheBiotinylated secondary antibodies can be used for tissue and cell staining, ELISAs, and Western blots. ThisBiotinylated Universal secondary antibody produced in horse binds equally well to both mouse IgG (H+L) and rabbit IgG (H+L) primary antibodies. The convenience of using a singleBiotinylated secondary antibody for both species makes it a simple to use reagent to expedite work flow, particularly in immunohistology applications using ReadyGo™ format.