We studied the immune responses of 33 patients with autoimmune thyroid disease (AITD; including 17 with Hashimoto's thyroiditis and 16 with Graves' disease), 5 patients with non-AITD, 12 control subjects (CS), and 2 subjects with a family history of autoimmunity to the main thyroid antigens. These antigens included thyroid peroxidase (TPO), thyroglobulin (Tg), TSH receptor (TSH-R), and 13 overlapping TPO peptides. T-cell lines (TCL) were isolated from peripheral blood mononuclear cells (PBMC) after incubation with TPO, Tg, or a protein derivative of tuberculin (PPD). PBMC and TCL were used in a 3- to 5-day microproliferation assay. Peripheral lymphocytes from most AITD patients responded with a stimulation index of 3 or more to TPO, Tg, and/or TSH-R (60-88%) as well as to two or more TPO peptides. Lymphocytes from 3 of 5 patients with non-AITD and 2 subjects with a family history of autoimmunity were also reactive to thyroid antigens. TPO TCL showed a high proliferative response to TPO and its peptides, whereas Tg TCL were less reactive and PPD TCL were nonreactive to these antigens. Six of the 13 peptides tested produced highly significant stimulation in PBMC (CS, 0-17%; AITD, 60-92%) and TPO TCL (73-91%). The amino acid sequences of these putative epitopes were located in TPO regions 100-119, 211-223, 261-275, 420-434, 625-644, and 882-901. These results demonstrate T-cell responses to the main thyroid antigens, including TPO, Tg, and TSH-R, and confirm the heterogeneity of TPO T-cell epitopes in patients with AITD. Amino acid residues 100-119, 420-434, 625-644, and 882-901 are the most common sites recognized by TPO TCL, indicating that they may be immunogenic epitopes in AITD.