Protein G, a bacterial cell wall protein with affinity for IgG, was isolated from a human group G streptococcal strain (G148). Bacterial surface proteins were solubilized by enzymatic digestion with papain. Protein G was isolated by sequential use of ion-exchange chromatography on DEAE-cellulose, gel filtration on Sephadex G-100 and affinity chromatography on Sepharose 4B-coupled IgG. The presence of protein G in various pools and fractions during the isolation was followed by their ability to inhibit the binding of radio-labeled IgG to G148 bacteria. A highly purified protein G was obtained. On polyacrylamide gel electrophoresis in sodium dodecyl sulfate, the apparent MW was 30,000; on agarose gel electrophoresis the purified protein gave rise to a single band in the .alpha.1-region. Protein G bound all human IgG subclasses and also rabbit, mouse and goat IgG. On the IgG molecule, the Fc part appears mainly responsible for the interaction with protein G, although a low degree of interaction was also recorded for Fab fragments. IgM, IgA and IgD, however, showed no binding to protein G. This novel IgG-binding reagent promises to be of theoretical and practical interest in immunologic research.