The effects of maternal-infant transfer of influenza-specific antibody on influenza infection in the infant mouse were studied by following mortality, hemagglutination inhibition antibody, virus shedding from the nose and lung, and pathology of the upper and lower airway. By cross-fostering infants, influenza-specific antibody was shown to be acquired mainly by breast-feeding. Mice acquiring specific antibody survived lethal influenza infection (p less than 0.001) and had no evidence of influenza pneumonitis. There was variable suppression of nasal virus shedding and tracheal desquamation. Infected infants that initially acquired high levels of serum antibody also showed no evidence for serum antibody 10 wk later. Infant mice surviving initial influenza infection were lethally rechallenged 10 wk later. Rechallenged mice had a higher survival rate when compared to control mice (p less than 0.001) in spite of the absence of anti-influenza antibody and the presence of high lung virus titers in both groups 1 day after rechallenge. Rechallenged mice showed evidence for both significantly lower nasal virus titers (p less than 0.001) and a secondary serum antibody response with associated decline in lung virus titer. These results suggest maternal influenza-specific antibody a) is transferred to the infant mouse; b) is transferred mainly by breast-feeding; c) is life saving and lowers lung virus titers in the infant; and d) suppresses the infant's serum antibody production after challenge without affecting local immunity or the ability to mount a secondary antibody response.