Neonatal mice succumbed to intranasally-inoculated Streptococcus pneumoniae doses which were as much as 250 times less than the doses that adult mice were resistant to. Neutrophil migration into lungs of neonates was similar in kinetics and intensity to that in adults in response to lethal doses of S. pneumoniae. Interestingly, neutrophil infiltration into the lung alveoli of neonates occurred at lower doses of bacteria than that required for similar responses in adults. Furthermore, depletion of neutrophils in adult and neonatal mice inoculated with low doses of bacteria resulted in significantly higher lung burdens of bacteria in neonatal mice as compared to adults. These data indicate that increased susceptibility of neonates to S. pneumoniae is not the result of incompletely developed neutrophil function and infact, indicate that neutrophils contribute more to resistance to low doses of S. pneumoniae in neonates than they do in adult mice.