$T_3-BSA;and;T_4-BSA$ conjugates were prepared and identified spectrophotometrically. The ${lambda}max$ of the conjugates was just coincided with that of BSA, but the molar extinction coefficients of the conjugates were generally larger than that of BSA itself. The molar ratios of $T_3:;BSA;and;T_4:;BSA$ in the prepared conjugates were found to be 9:1 and 5:1, respectively. The titers of the $T_3$ antisera were generally higher (max. $1.5{ imes}10^4:1$) than those of $T_3$ (max. $2{ imes}10^3:1$), and the average cross reactivity of the $T_3$ antibody with $T_3$ was lower (0.45%) than that of $T_4$ antibody with $T_3(3{sim}4%)$. The results of the study indicate that the predominant cause of the lower titers and the lower specificity of the $T_4$ antisera comparing with those of $T_3$ is mainly due to the unstability of the $T_4-BSA$ and consequent degradation of the conjugate to $T_3-BSA$ during preparation, purification, and even during immunization. The lower molar ratio of $T_4$ to BSA in the preparation stage is also considered to be a minor factor. By measuring $T_3,;T_4$ levels in the reference control serum, it has been confirmed that the prepared antisera can sufficiently be utilized, respectively, in the established radioimmunoassay systems.