Data from 4 trials involving 50 baby pigs are reported in which isolated soybean protein and casein were used in simulated milk diets to study the protein requirements of baby pigs during the period from 2 to 30 days of age. Diets containing 16, 20, 24, 28 and 32% of protein, on an air-dry feed basis, were studied. The diets with only 16 or 20% of protein always gave slowest growth rates and resulted in retarded animals. Fastest growth and most efficient feed utilization occurred on the 32% protein diet, but 24 and 28% of protein gave relatively satisfactory results. Casein provided a more satisfactory source of protein than did the isolated soybean protein supplemented with methionine.