Of the various physiological responses to prolactin the most characteristic and unequivocal is its ability to cause thickening of the crop-glands in doves and pigeons [Riddle & Braucher, 1931; Riddle, Bates & Dykshorn, 1933], and this response is now widely used for estimations of potency. Assay methods based on the crop-gland response fall into three main classes, (i) methods involving determination of the weight of the stimulated crop-glands as first proposed by Riddle et al. [1933], (ii) systemic minimal stimulation methods such as that of McShan & Turner [1936], and (iii) local minimum stimulation or 'intradermal' methods originated by Lyons & Page [1935]. The chief value of the micromethod of Lyons & Page is that it will detect minute amounts of hormone. For accurate assay, however, we prefer the crop-weight method to methods involving determinations of the minimal stimulation dose since the latter depends on a subjective criterion while crop-weight