We investigated factors that may contribute to lung infections in infants by studying the intrapulmonary responses to aerosols of three different types of organisms — group B streptococcus with and without type-specific capsule, Pseudomonas aeruginosa, and Staphylococcus aureus — in infant (12-h-old or 24-36-h-old) and adult (150 g, 6-w-old) rats. After aerosol exposure, the lung clearance rate of each organism varied inversely with the age of the animals, and the magnitude of the clearance defect was related more strongly to animal age than to the bacterial species. Fewer alveolar macrophages from infant animals phagocytosed each type of organism in vivo, and the rate of neutrophil accumulation in the lungs of infant animals was delayed. The neonatal lung functioned effectively, however, as an antibacterial barrier, as newborn animals survived an aerosolized inoculum that exceeded the LD₅₀ by the subcutaneous route.