Milk is generally accepted as being one of the rich natural sources of riboflavin. Since milk is readily available, it is commonly recommended as an excellent source of riboflavin for the human dietary. However, a careful inspection of the literature soon reveals that the riboflavin content of milk is extremely variable. For instance, Williams, Cheldelin and Mitchell1 reported that Jersey milk contained as little as 0.53 mg. of riboflavin per liter, while Lindholm2 found as much as 7.8 mg. in Jersey milk. In other words, the amount of riboflavin reported by Lindholm is nearly 1,500 per cent of that reported by the other authors even though both studies were conducted on milk produced by the same breed of dairy cows. Such data may well cause the physician to wonder concerning the riboflavin content of the milk that he recommends to his patients. Furthermore, Hand and Sharp3 found