A competitive fluorometric ELISA was developed which enabled pair-wise comparisons of the antigen-binding characteristics of thyroid microsomal autoantibodies (TMaab) and their antigen-binding fragments from different individuals. Antibodies rendered undetectable to the protein-A-enzyme conjugate by removal of their Fc-regions (F(ab')2's) were tested for their ability to inhibit the antigen-binding of intact thyroid microsomal autoantibodies (TMaab). Only F(ab')2's derived from TMaab-containing sera were capable of displacing TMaab, some being capable of complete displacement of allogeneic TMaab. The antigen-binding of Tgaab was unaffected, except by F(ab')2's derived from Tgaab-containing sera. The high degree of overlap observed in the specificity of TMaab suggests that different sera recognize the same epitopes. In this respect the autoimmune response to the microsomal antigen was shown to resemble that to thyroglobulin, which involves few epitopes. Cross-tolerance is postulated to be the mechanism underlying the tissue-specificity and epitope-restriction of thyroid autoantibodies.