HLA-A, B, C antigens were studied in 86 white european patients with Graves' disease, using a lymphocyte toxicity microtechnique and the results were compared with those obtained in 356 healthy subjects. HLA-D (DR) antigens were studied by the same technique after prolonged incubation and the results were compared with those of 100 healthy controls. The incidence of DRw3 was 51.16% in the patients as against 20% in controls, the difference being highly significant (pc--PC less than 0.0003) - corrected p = p multiplied by the number of antigens tested. There was also a significant (pc less than 0.001) increase in HLA-BB: 44.19% against 22.47%, and in HLA-A1: 40.7% against 28.93% (pc less than 0.03). Conversely, there was a decrease in the incidence of HLA-B12: 12.79% against 31.74% (pc less than 0.01). B8 was found to be associated with DRw3 in 37 of the 86 patients, but in only 13 of the 100 controls (p less than 0.00003). There was no correlation between the HLA antigens and the clinical features of the disease (presence or absence of goitre and exophthalmos, severity of clinical or biological symptoms). These results are in agreement with those of other studies reporting an increase in HLA-B8. The increase in HLA-A1 is probably due to an accentuation of the unbalanced linkage with B8. The major finding was the predominance of the DRw3 antigen, also found by other authors working with a mixed lymphocyte culture. It seems therefore possible that the putative Graves' disease susceptibility antigen is present on the sixth chromosome, near to HLA-D (DR).