Recombinant, enzymatically active human thyroid peroxidase (hTPO) generated in nonthyroidal eukaryotic cells was compared with Graves' thyroid microsomes as a source of antigen for the immunological detection of antimicrosomal/anti-hTPO antibodies. Enzyme-linked immunosorbent assay of 51 sera, selected to produce a balanced distribution of antimicrosomal antibody (anti-MSA) levels, revealed (at 1:100 serum dilution) a moderately good correlation between anti-MSA and anti-hTPO antibody levels (r = 0.668; P less than 0.001). However, a number of sera with high anti-MSA levels yielded markedly discordant values between the two assays. A much lower correlation was observed between antithyroglobulin and anti-hTPO antibody levels (r = 0.315; P less than 0.05). At higher serum dilutions (1:1,000 and 1:10,000), at which low affinity, high capacity binding reactions will be reduced, the correlation between anti-MSA and anti-hTPO antibody values was greatly improved (r = 0.906 and 0.902, respectively; P less than 0.001), and there were no longer widely discrepant values between the two assays. In summary, the present study indicates that recombinant hTPO expressed in nonthyroidal cells provides an unlimited source of human TPO of unvarying quality for anti-hTPO antibody assays. This material offers increased specificity over standard anti-MSA assays that use thyroid cell microsomes as antigen.