T3-BSA, T4-BSA conjugates were prepared and identified spectrophotometrically. The lambda-max on the conjugates was just coincided with that of BSA, but the molar extinction coefficients of the conjugates were generally larger than that of BSA itsalf. The molar ratios of T3:BSA and T4:BSA in the prepared conjugates were found to be 9:1 and 5:1, respectively. The titers of the T3 antisera were generally higher (max. 1.5×104:1) than those of T4 (max.2×103:1), and the average cross reactivity of the T3 antibody with T4 was lower (0.45%) than that of T4 antibody with T3(3∼4%). The results of the study indicate that the predominant cause of the lower titers and the lower specificity of the T4 antisera comparing with those of T3 is mainly due to the unstability of the T4-BSA and consequent degradation of the conjugate to T3-BSA during preparation, purification, and even during immunization. The lower molar ratio of T4 to BSA in the preparation stage is also considered to be a minor factor. By measuring T3, T4 levels in the reference control serum, it has been confirmed that the prepared antisera can sufficiently be utilized, respectively, in the established radioimmunoassay systems.