Biotinylated Protein A (B-2001), isolated from a special strain of S. aureus, is a 41 kDa protein which binds to the Fc portion of most immunoglobulins. This interaction is neither species specific nor isotype specific, as Protein A has been shown to bind IgG from a variety of species. Protein A can also interact with IgA, IgM and subclasses of IgG from different species.