The proliferative response of T lymphocytes when cocultured with autologous non-T cells in the absence of any other stimulating substance has been termed the autologous mixed lymphocyte reaction (AMLR). The AMLR has been shown to be impaired in several autoimmune disorders, such as systemic lupus erythematosus, Sjögren's syndrome, and primary biliary cirrhosis. In this study we report marked deficiency in the AMLR in two autoimmune disorders: Hashimoto's and Graves' diseases. This impaired AMLR, probably related to previously described T cell subset imbalances, provides further evidence of altered interactions between the immunocompetent cellular subsets in patients with these pathological conditions. Additional preliminary observations suggest defective allogeneic mixed lymphocyte reactions as well.