ExtractNine Sicilian children known to be deficient in glucose‐6‐phosphate dehydrogenase (G6PD) were studied to see if there were anomalies of bactericidal activity in peripheral blood phagocytes. The type of deficiency was established. The G6PD levels in the leukocyte were found to be 26% of the controls (0.094 ± 0.03, normal controls 0.360 ± 0.12). The Michaelis constant for NADP and glucose‐6‐phosphate (G6P) was lower than the control. Conversely, the utilization of the analogous 2‐deoxyglucose‐6‐phosphate (2dG6P) and galactose‐6‐phosphate (Ga16P) was higher. The thermostability of the enzyme in the deficient subjects was lower and the pH optima (8 and 9.5) were different from the controls. An identical electrophoretic pattern was found in both normal and deficient subjects. The bactericidal activity in the deficient subjects was normal. There was no difference in the results of nitroblue tetrazolium (NBT) tests in either group.SpeculationAlthough leukocyte G6PD was only one‐quarter of the normal level, the phagocytic activity and the NBT test were normal in all subjects studied. It is not clear how such low levels of enzyme allow normal function. Perhaps further investigation under simulated intracellular conditions could give more reliable information about the enzyme activity.