The B1 domain of streptococcal protein G interacts with the C-terminal fragment of the heavy chain of immunoglobulin G (IgGFc). The binding site for the protein G domain on the antibody fragment is in close proximity or overlapping with that determined for staphylococcal protein A. The interaction of the B1 domain with IgGFc was investigated by 1H-15N correlation spectroscopy. The major interaction site on the B1 domain comprises parts of beta-strand 3 as well as the alpha-helix. Comparison with the crystal structure of the protein A/IgGFc complex suggests that the mode of interaction in the two complexes is analogous, despite the lack of sequence or structural similarity between two antibody binding proteins.