Patients with Graves' disease were phenotyped for properdin factor B (Bf) and glyoxalase, which are coded for by genes mapping close to the HLA region on the sixth chromosome. Frequency data were analysed in relation to HLA-A, -B and -DR typing data. Diagnosis of Graves' disease was based on the usual criteria including elevated T3 and T4 levels and free T4 index and a homogeneous thyroid scan. Ninety-four patients with Graves' disease were phenotyped for properdin factor B (Bf) and 37 for red cells glyoxalase (GLO). HLA-A, -B and -DR antigens were typed in 94 patients using a lymphocyte microcytotoxicity assay. The frequency distribution of Bf and GLO alleles showed no significant differences from control subjects. This finding contrasts with the reports of an increased frequency of BfF1 in insulin-dependent diabetes mellitus. The difference in the two diseases which are both associated with an increased frequency of the antigen combination D8-DR3, is accounted for by linkage disequilibrium between B18 and BfF1.