Five sources of protein have been studied with respect to their ability (1) to maintain normal hemoglobin levels in the growing rat and (2) to support hemoglobin regeneration after hemorrhage in the adult rat. The various preparations were fed to weanling animals at both 18% and 2.8% levels of protein for an experimental period of approximately 200 days. At the 18% protein level, casein, lactalbumin, dried skim milk, and a mixture of dried skim milk and dried beef blood proved to be of about the same value, both for hemoglobin maintenance in the growing rat and for hemoglobin regeneration in the adult animal. On the other hand, dried beef blood protein was inferior, particularly for hemoglobin regeneration after hemorrhage. There appeared to be a general parallelism between the hemopoietic value of the protein and its ability to support somatic growth. At the 2.8% level of the various proteins, a mild to moderate anemia developed in all cases. However, only those animals receiving lactalbumin survived for the entire experimental period. In these animals, there was a significantly decreased rate of hemoglobin regeneration following hemorrhage. However, there was a very efficient utilization for hemoglobin regeneration of the limited amount of protein consumed. These observations emphasize the importance of both the quality and quantity of dietary protein for hemopoiesis in the rat.