The effect of disulphide bridges reduction of the microsomal antigen (Mic-Ag) and thyroid peroxidase (TPO) by dithiotreitol (DTT) has been investigated. The reaction of all 67 tested sera from untreated hyperthyroid Graves' and from 22 Hashimoto's patients with high microsomal antibodies (aAb) titer was diminished by 90–95% by DTT, at pH 9.6. The remaining S-10%of the activity was not destroyed by DTT. The residual Mic-Ag after DTT reduction was able to inhibit the binding of all 45 Graves' and 22 Hashimoto's tested aAb's to the native microsomal antigen by 100% at high concentration. Reaction of affinity purified TPO with two monoclonal antibodies (mAb) were diminished by 80% to 95% by DTT pretreatment, while the reaction of one mAb with TPO was only slightly affected. The reaction of TPO and Mic-Ag with rabbit polyclonal anti-TPO serum (rabbit aTPO) was diminished by 60% by DTT pretreatment. The immunological reactivity of TPO with aAb's was diminished by 65% after DTT pretreatment. The microsomal antigen-aAb's complex was not destroyed by DTT. Results presented in this paper suggest conformational epitope structure of the Mic-Ag recognized by aAb's in patients with thyroid autoimmune disease (AITD).