The respiratory burst of human neutrophils was measured under conditions of hypoxia and low pH. O2 -- production by neutrophils activated with opsonized zymosan fell slowly as the oxygen concentration declined to 1%, then dropped more sharply, reaching negligible levels at oxygen concentrations less than 0.25%. Production was half maximal at an oxygen concentration of 0.35% (equivalent to approximately 10-microM dissolved oxygen). O2- production by the cell- free O2- -forming system prepared from zymosan-activated neutrophils showed a similar dependence on oxygen concentration. A drop in pH caused decreases in both oxygen consumption and O2-- production by zymosan-treated neutrophils, values at PH 6.0 being 10%--20% of those observed at pH 7.5. Experiments with the cell-free O2-- -forming system suggested that this decline in respiratory burst activity at low pH was due to inefficient activation of the O2-- -forming enzyme under acidic conditions.