The existence of testosterone-binding globulin (TBC) homogeneous by the association constant (of the 10(8) M-1 order) and sedimentation characteristics (sedimentation coefficient--4S) was demonstrated in various groups of rabbits (mature males and females, pregnant females and fetuses, immature males). The level of the TBG binding capacity failed to show any marked differences in all the ontogenetic groups. The TBG level was rather close in the rabbit mother and fetus. Estradiol, estron, progesterone, cortizol possessed no affinity to this protein. Dehydrotestosterone affinity was about three times greater than that of testosterone. When TBG was used in the competitive protein-binding analysis of testosterone and other protein-bound androgens in the blood plasma of men, women and rabbits (male and female) there were obtained results coinciding with those of other authors. This permitted to recommend TBG for wide practical use in the competitive protein-binding analysis.