SummaryThis study compared the bactericidal capacity of polymorphonuclear leukocytes (PMNs) from neonates and adults for type Ic group B Streptococcus (GBS), and examined the effect of severe stress on the bactericidal capacity of PMNs from newborn infants. PMNs were obtained from three study groups: 26 adults, 13 healthy neonates (cord blood), and 29 stressed neonates. Stress was defined as an acute respiratory illness or bacterial infection requiring assisted ventilation. Bacterial killing was assessed using a fluorochrome microassay and PMNs adherent to glass coverslips. PMNs from stressed infants killed significantly fewer GBS than PMNs from adults (P<0.001 at both time points). PMNs from healthy infants also demonstrated reduced killing compared with adults (P<0.01 at 60 min; P<0.001 at 90 min). There was no significant difference in bacterial killing between stressed and healthy neonates and no correlation between bactericidal capacity and age at time of study, gestational age, birth weight, peripheral leukocyte count, or Apgar scores. Therefore, the bactericidal capacity for GBS by PMNs from neonates is diminished; however, it is not further compromised by stress.