We have studied by flow cytometric analysis the antigen specific activation of CD4+ (helper/inducer) T lymphocytes by purified human thyroid peroxidase (TPO). Peripheral blood mononuclear cells were obtained from 26 patients with Graves' disease (GD), 16 with Hashimoto's thyroiditis (HT), 7 with nontoxic nodular goiter (NG), and 14 normal subjects (N). Cells were cultured for 7 days in the presence or absence of TPO at final concentrations of 3, 30, and 300 ng/mL. When harvested, cells were reacted with an FITC-conjugated anti-CD4 and a PE-conjugated anti-HLA-DR murine monoclonal antibodies. The percentage of HLA-DR+ CD4+ cells (activated CD4+ cells) was determined by a flow cytometer. In the absence of TPO, CD4+ cells had been activated without any specific stimulant. This is known as the autologous mixed lymphocyte reaction (AMLR). In the AMLR, CD4+ cells from GD and HT were less activated compared to those from NG and N. Results of TPO-specific activation were expressed as an incremental increase of activated CD4+ cells (II) (percentage of activated CD4+ cells cultured with TPO minus percentage of activated CD4+ cells cultured without TPO). II of N, GD, HT, and NG were 0.37 +/- 0.21, 2.20 +/- 0.45,** 2.0 +/- 0.66,* and 0.35 +/- 0.27 (mean +/- SEM), respectively (**p less than 0.01; *p less than 0.05 vs N). When patients were further subdivided, the highest mean II was found in patients with hyperthyroid GD (p less than 0.01), followed by euthyroid HT (p less than 0.05) and euthyroid GD (p less than 0.05), however there was no significant difference between hypothyroid HT and N. In conclusion (1) AMLR reactivity of CD4+ cells from GD and HT was impaired, (2) however, CD4+ cells from both GD and HT were significantly more induced by TPO compared to N, and (3) this induction depends, in part, on the in vivo thyroid status.