Sixty two thyrotoxic patients, 34 African and 28 Indian, were studied in order to assess the prevalence of thyroid antibodies and TSH binding inhibitory activity (TBI): 45 had Graves' disease and 17 had toxic nodular goitres. Microsomal and thyroglobulin antibodies were positive more often in Indian than in African patients with Graves' disease (microsomal 52% vs 37.4%, P less than 0.05; thyroglobulin 38.1% vs 4.2%, P less than 0.001). Patients with toxic nodular goitres had a lower prevalence of positive microsomal antibodies (P less than 0.01), but not of thyroglobulin antibodies (P = 0.1) when compared with patients with Graves' disease. TBI activity measured by a radioreceptor assay was positive in 43 of the 45 (95%) patients with Graves' disease and only 1 of the 17 patients (5.9%) with toxic nodular goitre. It thus appears that TBI activity is a sensitive marker in the diagnosis of Graves' disease and that there is a lower prevalence of thyroglobulin and microsomal antibodies in African patients compared with Indian patients.