Polymorphisms within the immunoglobulin heavy chain constant region at switch (S)mu and S alpha 1 loci were studied in patients with rheumatoid arthritis (RA) and controls in parallel with Gm and A2m allotyping. S mu and S alpha 1 restriction fragment length polymorphisms were defined using a S mu probe which was hybridised to SstI digests of DNA extracted from circulating white cells. There were no differences in S mu and S alpha 1 gene or phenotype frequencies between RA and control groups nor within the RA population between DR4 positive and negative subjects. The Gm allotype G1m(x), coded for by genes at the gamma-1 locus, was found in 29/92 (32%) of DR4 positive and 5/52 (10%) of DR4 negative subjects with RA, as compared with 25/115 (22%) cf controls. The A2m allotype A2m(2), coded for by the alpha 2 locus, was found in 11/92 (12%) of DR4 positive subjects with RA, zero of 52 DR4 negative subjects, and 5/115 (4%) of controls. These results exclude a major effect of genes within the heavy chain constant region linked to mu or alpha 1 loci on susceptibility to RA, but suggest that further study of variants closely linked to the alpha 2 locus is indicated.