1. Erythropoiesis in birds is stimulated by bleeding and environmental hypoxia and is suppressed by induced polycythemia, indicating that the physiologic response is similar to that in mammals.
2. This response is mediated through a humoral factor which stimulates erythropoiesis of birds but not of mammals. Similarly, the erythropoietin of mammals does not stimulate the erythropoiesis of birds.
3. Although both the erythropoiesis stimulating factor of birds and of mammals appear to require an intact protein structure for biological activity, the erythropoietin of birds differs from that of mammals in that it is not destroyed by sialidase and does not lose its activity when reacted with antibody to human urinary erythropoietin.