OBJECTIVE: To investigate the influence of neonatal maturity on active oxygen metabolism in neutrophils and possible causes of a high susceptibility to bacterial infection in preterm infants. METHODS: Thirty-five preterm infants born at a gestation age of 26-32 weeks (≤32 weeks group, n=15) and at 33-36 weeks (> 32 weeks group, n=20) and 23 full-term infants (control group) were enrolled in this study. The samples of whole cord blood from the two preterm groups and the control group were stimulated in vitro with live bacteria,Staphylococcus aureus( S. aureus) and Escherichia coli (E. coli) and stained with hydroethedine, an indicator of superoxide. The percentage of neutrophils which produced superoxide and the mean fluorescence intensity for superoxide production were measured by flow cytometry. The incidence of bacterial infection during hospital stay was compared between the two preterm groups. RESULTS: Under S. aureus or E. coli stimulation, the percentage of neutrophils which produced superoxide in the ≤32 weeks group was significantly lower than that of the > 32 weeks group and the control group (P<0.01). The percentage of neutrophils which produced superoxide was closely related to gestational age in preterm infants ( y＝2.66 x, P＜0.01).There were no significant differences in the blood level of superoxide production in neutrophils among the three groups. The incidence of bacterial infection during hospital stay in the ≤32 weeks group (40%) was significantly higher than that the > 32 weeks group (10%) (P<0.05). CONCLUSIONS: The capability of active oxygen metabolism in neutrophils was significantly related to the gestational age in preterm infants. The decreased capability of active oxygen metabolism might be contributed to a higher susceptibility to bacterial infection in preterm infants.［Chin J Contemp Pediatr, 2007, 9 (4):355-357］