Purified, recombinant M12 protein from Streptococcus pyogenes CS24 has recently been demonstrated to bind human immunoglobulin G3 (IgG3). The binding site for IgG has been localized to an internal peptide encoded by a PvuII fragment of the gene emm12. We have investigated the ability of an isolated recombinant M12 protein consisting of the peptide encoded by the PvuII fragment to bind various monoclonal human IgG3 myeloma proteins representing a number of both Caucasian and Oriental IgG3 Gm(allotypic) phenotypes. Of nine Caucasian IgG3 myeloma proteins, only two bound strongly to the recombinant M12 protein in enzyme-linked immunosorbent assays. The allotypic phenotypes of the reactive proteins were IgG3m(b+)(g-) and IgG3m(b-)(g+). No binding was seen for seven IgG3 myeloma proteins of Oriental origin with G3m(st+)(u-)(b+)(g-), G3m(st-)(u+)(b+)(g-), G3m(st-)(u+)(b-)(g+), and G3m(st-)(u-)(b-)(g+) phenotypes. The binding of human IgG3 to M12 protein seems to be related to features other than its Gm allotypic markers. Selective reactivity of IgG3 myeloma proteins with M12 protein may provide another way to subclassify human IgG3 molecules. The biological significance of the selective reactivity is not known.