Protein A, isolated from a special strain of Staphylococcus aureus, is a 41,000D protein capable of binding to the Fc portion of immunoglobulin. This interaction is not species specific, as Protein A has been shown to bind IgG from a variety of species. The binding was thought to be restricted to IgG, but subsequent studies have demonstrated that Protein A can interact with IgA, IgM and subclasses of IgG from different species. Biotinylated Protein A is a useful reagent for detecting or localizing immunoglobulins on cell surfaces or in tissue sections. Used with the appropriate Avidin D conjugate, immunoglobulins can be studied at the fluorescent or electron microscopic level. Biotinylated Protein A can also be employed for the detection of positive clones in monoclonal antibody production by using Avidin D conjugated with peroxidase or other marker enzymes. Used in many systems as a substitute for a biotinylated second antibody. Because Biotinylated Protein A may produce a high backgr