Serum antibodies against human, bovine, and guinea-pig, orbital antigens were tested for in patients with Graves' ophthalmopathy using haemagglutination and immunofluorescence techniques. Although low titres of antibodies were demonstrated in a small proportion of patients with Graves' ophthalmopathy, the prevalence was similar to that for normal subjects using both tests. The significance of these findings and the evidence for a role of autoantibodies in the pathogenesis of Graves' ophthalmopathy are discussed.