It is an established fact that the mineral constituents of cow's milk are not so well utilized by infants as are those of breast milk. This fact, however, seems to have been based largely on investigations in which different infants were employed as subjects for the experiments on the two kinds of milk. It is a matter of surprise, on going through the literature, to see how little has been written regarding the mineral metabolism in the same infant fed on the two kinds of milk. Since the constitution of normal people is known to vary to a certain extent with different persons, the difference in the utilization of the mineral constituents of the two milks may be owing not only to the nature of the milk, but to the constitutional differences of the individual infants. In order to determine whether there is any difference in the utilization of mineral