It has long been recognized that the pleomorphic changes which occur in the pituitary gland during estrogenic cycles in the experimental animal are probably associated with hormonal activity. While various histochemical staining techniques demonstrate the particular cells concerned in these changes, the presence of a particular hormone cannot be directly demonstrated by these methods. Using ovine prolactin as an antigen with Freund's adjuvant, it has been possible to produce precipitating antisera capable of inhibiting the action of prolactin on the mucosa of the pigeon crop sac. By means of fluorescent anti-prolactin globulin, prolactin has been demonstrated in the cytoplasm of the acidophilic (α) cells of the pars distalis of the cat and rat pituitary.