Conclusions and Summary 1. Once a day for 4 days 7 normal rabbits each received 50 ml of serum obtained from hypoxic rabbits, and 6 normal rabbits each received the same amount of serum obtained from normal rabbits. The hypoxic serum induced a reticulocytosis which was significantly different (p value < 0.01) from the response induced by injection of normal serum. At the level of hypoxia used (10% oxygen for 48 hours) the response in the recipient animals was as great as that seen in the animals directly exposed to hypoxia. 2. These results bolster the hypothesis that the erythropoietic response to hypoxia, or to blood loss anemia involves the production of a humoral factor.