Treatment of rabbit mammary membrane-bound and solubilized prolactin receptors with the oxidizing agents N-chlorobenzene sulfonamide and N-bromosuccinimide resulted in total inactivation of the ability of the receptor to bind prolactin. A similar inactivation was obtained with 2-hydroxy-5-nitrobenzylbromide. The results suggest the possible importance of tryptophan in maintaining the activity of the prolactin receptor.