Protein G is a bacterial cell wall protein expressed at the cell surface of certain group C and group G Streptococcal strains. It has affinity for both Fab- and Fc-fragments of human IgG by independent and separate binding sites. Binding to the Fc region of immunoglobulins from several species by a non-immune mechanism exhibits great affinity for almost all mammalian immunoglobulin G (IgG) classes, including all human IgG subclasses (IgG1, IgG2, IgG3 and IgG4) and also rabbit, mouse, and goat IgG. Protein G bound all tested monoclonal IgG from mouse IgG1, IgG2a, and IgG3, and rat IgG2a, IgG2b, and IgG2c. In addition, polyclonal IgG from man, cow, rabbit, goat, rat, and mouse bound to protein G, whereas chicken IgG did not. Protein G has also been shown to bind human serum albumin but at a site that is structurally separated from the IgG-binding region. Protein G shows a broader range of binding to IgG subclasses than staphylococcal protein A. This applies to polyclonal IgG from cow, rat, goat, human and rabbit sources as well as several of rat and mouse monoclonal antibodies. In contrast, protein A shows stronger interaction with polyclonal IgG from human, guinea-pig, pig, dog and mouse. Both proteins interacted with same relative strength to polyclonal rabbit IgG. Protein G consists of nearly 600 amino acid residues. The carboxy-terminal half contains three immunoglobulin G (IgG)-binding domains which are referred to as domains I, II, and III or units C1, C2 and C3, each containing 55 amino acid residues with two 'spacers', of 16 amino acids, Dl and D2. Following the IgG-binding regions there is a region W, which most likely is involved in cell wall interactions. Domains in the NH2-terminal half of the protein have been found to bind human serum albumin (HSA).