When the substitution of a food containing cow's milk for a food not containing it is followed by vomiting and diarrhea, with perhaps other symptoms, it is easy to assume an idiosyncrasy which causes all food containing cow's milk, however intelligently administered, to act like a poison. In all probability, in a considerable number of the cases reported in the literature as examples of hypersensitiveness to cow's milk and a great proportion of those so regarded and treated in practice, the patients suffered merely from ordinary forms of indigestion. Hypersensitiveness to cow's milk has been made the subject of actual proof, 1 like the hypersensitiveness to egg, 2 to ragweed pollen, 3 to pork, 4 and to other substances through its transference to animals by injection with the patient's blood serum. Strong corroboratory evidence of its existence can be obtained if precipitins are found in the blood, 5 or by