Thyroid peroxidase (TPO) is a member of the thyroid autoantigens. Anti-thyroid autoantibodies, anti-TPO,-thyroglobulin and -TSH receptor autoantibodies, are important in diagnosing autoimmune thyroid diseases and for judging treatment efficacy. To diagnose chronic thyroiditis, anti-TPO autoantibody detection is the most sensitive among the three anti-thyroid autoantibodies. Eighty-five to ninety percent of patients with chronic thyroiditis have anti-TPO autoantibodies. Using recombinant human TPO protein, a kit measuring anti-TPO antibodies specifically have been developed. The developmental process and specificity of the kit are discussed. Compared with the microsome test, the most greatest advantage of the anti-TPO EIA kit can specifically measure anti-TPO antibodies. Structure of anti-TPO antibody immunoglobulin, which has been demonstrated over the last several years using the phage display method, is also discussed. The most prominent feature is that VL is believed to be important in determining anti-TPO antibody specificity in contrast to that of other autoantibodies, although VL itself does not have TPO-binding activity.