1. Young male albino rats were subjected to weekly bleedings by cardiac puncture for eleven weeks. Phagocytic activity of the granulocytes in this blood was determined at each bleeding by a whole blood in vitro technic and compared to that of the blood from a control group consisting of 5 animals. Total blood cell counts and hemoglobin determinations were also made each week with both groups. The control animals were not bled more frequently than once in six weeks.
2. Phagocytic function in the bled animals increased approximately 40 per cent above the values for the controls during the period of weekly bleeding. There was no evidence of anemia developing during this period.
3. The experimental animals were then bled three times weekly for five weeks. Phagocytosis increased about 80 per cent above control values and the anemia became severe. Macrophages obtained from a peritoneal exudate stimulated by injections of mineral oil at the end of this five week period became approximately twice as phagocytic as macrophages from control animals.
4. With the cessation of bleeding, there was a complete remission of the anemia as measured after two weeks and the phagocytic activity of the blood had dropped below normal. There was a partial return to normal within four weeks, but it was still slightly subnormal at six weeks. The macrophages, on the other hand, were normal when tested four weeks after recovery.
5. Control animals subjected to ether anesthesia thrice weekly for six weeks showed normal phagoctyic functions, as measured with blood and macrophages.
6. These results are discussed in the light of earlier experiments on the phagocytic activity of blood from anemic humans.