1. The nature of the increased phagocytic activity of the neutrophiles in anemic blood as measured in vitro is investigated. The reliability of the method is discussed and ways of improving the accuracy are described.

2. Whole normal blood was diluted to anemic levels with physiological saline alone or with additional homologous serum in order to test the effect of mechanical interference of a variable number of cells on phagocytosis. The results are equivocal. Reducing erythrocyte numbers to anemic levels by removing the packed red cells in centrifuged blood and then resuspending the remaining cells and serum showed that no change in phagocytosis was produced by this means.

3. Data on varying the oxygen tension in the atmosphere above the phagocytic system showed no consistent effects except a possible elevation in phagocytosis in pure oxygen. The time of exposure to the gases varied from zero to five hours before the addition of the bacterial suspension to the system.

4. Washed leukocytes from anemic blood had the same elevated phagocytic activity when returned to either anemic serum or to normal serum, while washed normal leukocytes had the same phagocytosis in both normal and anemic serum. It was therefore concluded that the change leading to an increased phagocytosis in anemia occurs in the neutrophile rather than in the serum.

5. These findings are discussed in the light of additional data now being prepared for publication.

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