Clinical observations indicate the possibility that there is an immunologic relationship between human, cow's and goat's milks. It is a matter of common experience that eczema and other manifestations of hypersensitiveness in the nursing baby are frequently aggravated by the addition of cow's milk, and conversely that eczema in the milk-sensitized infant is unrelieved by reverting to human milk or by the substitution of goat's milk. The work of Fleischer1 Bauer,2 Wells3 and von Versell4 suggested that this interrelationship may reside in a biologic similarity of the caseins of the three milks. The establishment of this fact seemed to be of sufficient importance, because of its practical application, to warrant further investigation. With this end in view, we undertook a series of experiments in which we used purified cow's, human and goat's caseins. The results of this work form the basis of the present communication.