Specificity of progestin binding sites for ovine mPRα. Binding of 3H-progesterone was measured in the absence or in the presence of increasing concentrations of nonradioactive progestins. Progestins evaluated included progesterone (P4), 20α-hydroxyprogesterone (20-OH-P4) and 17α-hydroxyprogesterone (17-OH-P4). Each point represents the average ± S.D. of progestin binding expressed as a percentage of maximum 3H-progesterone binding.