Two rabbits (TG-1, TG-2) were immunized with human thyroglobulin (HTg) and bled serially. Antisera were obtained at different times after the first immunization and kept separately and studied. In both rabbits production of anti-HTg, and anti-thyroid hormone antibodies such as anti-thyroxine (T4) and anti-triiodothyronine (T3) antibodies was observed. Binding parameters of anti-HTg antibodies with HTg, T4, and T3 were calculated in two selected antisera (70-day and 249-day). The Scatchard's plots of these antibodies were all curve-linear and were analyzed in two components: one, higher binding constant (Ka1) and smaller binding capacity (Cap1) and the other, lower binding constant (Ka2) and larger binding capacity (Cap2). Ka1 values of anti-HTg, anti-T4, and anti-T3 antibodies in sera from TG-1 obtained from 70-day and 249-day bleeding were 1.1 X 10(10) M-1, 6.0 X 10(9) M-1. 7.9 X 10(8) M-1 and 1.7 X 10(10) M-1, 6.5 X 10(9) M-1, 1.0 X 10(9) M-1, respectively. Those from TG-2 were 1.7 X 10(10) M-1, 1.8 X 10(9) M-1, 6.4 X 10(8) M-1 and 2.0 X 10(10) M-1, 3.1 X 10(9) M-1, 1.6 X 10(9) M-1, respectively. The significance of the production of anti-HTg and anti-thyroid hormone antibodies in rabbits immunized with HTg in relation to the antigenic structure of HTg molecule was discussed.