The antibody-dependent cell-mediated cytotoxicity of peripheral lymphocytes from patients with Graves disease was tested using chicken erythrocytes coated with rabbit antichicken red blood cell antibody and labelled with 51Cr. An increased cytotoxic activity was found in untreated Graves disease without ophthalmopathy. The cytotoxic activity was less elevated in the groups of patients with severe ophthalmopathy or high level of anti-thyroglobulin antibody, and approximately normal after methimazole treatment.