In high-salt medium, cytosol from immature rat uteri displays two main high-affinity estradiol-binding peaks after ultracentrifugation in a sucrose gradient. The two components are the estradiol receptor which has a sedimentation coefficient of 5.5 S, and the alpha-fetoprotein which sediments at 4.5 S. The dissociation rate constants (k-1) of plasma alpha-fetoprotein-estradiol complexes measured at 0 degrees in the absence or presence of 0.4 M KCl were found to be 7 X 10(-5) and 8 X 10(-5) sec-1, respectively. The half-time of dissociation of these hormone-plasma protein complexes is 100-200 times more rapid than that of the estradiol-receptor complexes. These data led to the use of two "differential dissociation" methods for the measurement of the hormone-binding protein complexes. In a high-salt cytosol, the charcoal technique measured selectively the receptor binding sites; the hydroxylapatite technique measured the sum of the alpha-fetoprotein plus receptor binding sites. Under these conditions, binding specificity studies provided evidence that alpha-fetoprotein is not a subunit of the receptor. This was confirmed by binding specificity studies in high-salt medium of the receptor separated from alpha-fetoprotein by ultracentrifugation.