Summary Eighteen cows were used in a study of the effect of thyroprotein on milk composition. They were fed 15 grams daily of the hormone for periods of six weeks. The milk was sampled at the start and twice during the feeding-period. Determinations made were: total solids, fat, ash, total protein, and casein. Lactalbumin and globulin, and lactose were determined by difference. Most important changes noted here were a considerable and rather consistent decrease in casein, and a roughly proportional increase in lactalbumin and globulin. Changes in fat content of the milk were not consistent, an increase being noted in only about half of the cases. Variations in milk composition between individuals within a breed in the great majority of cases were greater than variations between breeds. Wide variations in milk yield response to the hormone were also noted. These results should be considered as preliminary. It is planned to repeat the trials during the coming winter in a more intensive manner.