A culture of Streptococcus dysgalactiae (C 26) was shown to bind only to 125I-IgG, whereas another S. dysgalactiae culture (C 12) bound both 125I-IgG and 125I-albumin. The IgG-binding proteins could be readily solubilized by lysozyme treatment of the bacteria and isolated by affinity chromatography on IgG Sepharose. The purified IgG-binding protein from S. dysgalactiae C 26, which lacked simultaneous albumin binding activity, precipitated with IgG preparations from man, cow, horse, pig and mouse but not with chicken IgG. This IgG-binding protein was coupled to CNBr-activated Sepharose and subsequently used for the purification of IgG from both bovine and human serum. SDS-PAGE and immunoelectrophoretic studies confirmed the purity of the eluted proteins.