Seventeen reactive dyes were separately covalently attached to Sepharose-4B and examined for their ability to subfractionate the immunoglobulin G found in human serum. Each dye-Sepharose-4B adsorbent appeared to bind a characteristic proportion of the protein when tested with "pure" preparations. It is suggested that this is due to stereospecific interactions between each dye and various interaction sites on the immunoglobulin G molecules. These sites of interaction are not those which define the four major subclasses of the H-chain nor those which distinguish the kappa and lambda light chains. The adsorption may be influenced by the presence of other proteins when serum samples are tested due to competition for binding to the immobilised ligands but sequential use of the adsorbents can progressively enrich a particular immunological activity in human serum. This method offers a convenient and gentle way to subfractionate immunoglobulin G especially when the antigen is not known or unavailable for conventional affinity chromatography.