This chapter discusses affinity labeling (AL) of hormone binding sites. AL is a technique for covalently labeling protein binding sites. In this method, a reactive functional group is incorporated within the structure of a ligand molecule in such a manner that the modified ligand is bound by the protein and reacts with amino acid residues in or near the binding site, forming a covalent bond through the attaching function. The salient feature of an AL process is that covalent bond formation takes place within a complex of protein and labeling reagent and the high, localized concentration of reagent in the complex results in an enhanced reaction between the reagent and nearby amino acid residues, which is the basis for the labeling selectivity. The labeling reagent, particularly if non-polar, may associate in a low-affinity, non-specific manner with hydrophobic regions of other proteins, and labeling can take place through this complex. 16α- and 11α-Bromoacetoxyprogesterone and 20-acryloxyprogesterone irreversibly inactivate the 11β- and 21-hydroxylase activities in the rat adrenal and progesterone can protect against the effect of the 16α and 20-derivatives.