1. The metabolism of oxygen by phagocytosing neutrophils was traced by using 15O2.
2. The isotope did not exchange with the incubation medium or cells to an appreciable extent and unmetabolized oxygen was readily eluted by gassing the cell suspension.
3. The polarographic measurements of oxygen consumption closely paralleled the recovery of metabolized 15O2.
4. Almost all the metabolized 15O2 was converted into water, both in the presence and absence of KCN, supporting the concept that the oxygen consumed by neutrophils is converted into H2O2. It is unlikely that an appreciable proportion of this oxygen is incorporated into the organic composition of the cell or of the ingested microorganism.