Two distinct steroid-binding proteins are present in rabbit plasma. One of the proteins (TeBG) binds [3-H]5 alpha-dihydrotestosterone (5 alpha DHT) and [3-H]testosterone. The affinity of this binding protein for 5 alpha DHT was 3-4 times greater than for testosterone. Binding of [3-H]5 alphaDHT could be inhibited by unlabeled 5 alpha DHT, testosterone, 5 alpha-androstan-3 alpha,17 beta-diol (3 alpha-diol), and 17 alpha-methyl- B-testosterone (skf) 7690). The relative affinity of the competitors was: 5 alpha DHT greater than 3 alpha-diol greater than testosterone greater than SKF 7690. The antiandrogens, cyproterone (1,2 alpha-methylene-6-chloro-pregn-4,6-diene-17 alpla-ol-3,20 dione), cyproterone-17-acetate, and 6 alpha-bromo-17 beta-hydroxy-17 alpha-methyl-4-oxa-5 alpha-androstan-3-ine (BOMT) were ineffective in competing for [3-H5d alpha DHT binding sites, as were 4-androstene-3, 17-dione, 17 beta-estradiol (E2), progesterone, and cortisol. The formation of the [3-H]5 alpha DHT-TeBG complex was extremely rapid; the binding reaction was essentially completed in 15 s. The complex dissociated rapidly in the presence of charcoal. The dissociation rate constant (Kdiss) was 0.157 min- minus 1 and the dissociation half-time t-1/2) was 4.5 min. In the presence of charcoal and unlabeled 5 alpha DHT the Kdiss was 0.268 min- minus 1 and the t=1/2 was 2.6 min. The sedimentation coefficient of TeBG was congruent to 4.6 S and its molecular weight, estimated by gel filtration on a calibrated Sephadex G-200 column, was congruent to 75,000. The concentration of TeBG in male rabbit plasma decreased with sexual maturation and was approximately three times higher in adult females than in adult males. The other protein (CBG) bound both [3-H]cortisol and [3-H]progesterone. Binding of these compounds could be inhibited by unlabeled cortisol and progesterone, but not by unlabeled 5 alpha DHT, testosterone, or E2. CBG had a sedimentation of congruent to 3.9 S and an apparent molecular weight of congruent to 105,000. TeBG could be separated from CBG by a 60% ammonium sulfate precipitation and by gel filtration chromatography. Both proteins are thermolabile; TeBG is inactivated at temperatures above 30 degrees C and CBG is inactivated at temperatures above 50 degrees C.