On Southern blotting of DNA extracted from thyroid glands of five patients with Graves' disease, two probes (720 bp and 942 bp) for gag human immunodeficiency virus type 1 (HIV-1) gave a positive hybridisation signal in all samples tested. DNAs from peripheral blood mononuclear cells hybridised with the 720 bp gag HIV-1 probe in three of the five patients, none of whom had antibodies to HIV-1. Negative results were obtained with DNA from normal thyroid glands, thyroid neoplasms, various unrelated normal tissues, and virus-infected human cell lines. The intensity of the signal and the pattern of bands observed with the DNA of Graves' patients were heterogeneous and, in general, were not the same in the thyroid glands and peripheral blood mononuclear cells of individual patients. Similarly, no correlation was found between the positive hybridisation signals and other genetic and immunological indices or the duration of anti-thyroid drug treatment at the time the patients were investigated. The findings suggest the presence of a novel retrovirus, and the retrovirus-like sequences seem to be closely associated with thyroid autoimmunity.