Transplacentally acquired antibody to influenza A virus was measured by a microneutralization test and a radioimmunoprecipitation assay in cord blood obtained from infants at a large urban county hospital in 1975-1978. Random samples tested before epidemic periods were a measure of susceptibility of the population. Twenty-six infants from whom cord sera were available had culture-documented infections with influenza A/Victoria (H3N2) virus when younger than four months. The direct correlation between age at the time of infection and level of antibody measured in cord serum (P less than 0.002) suggested a protective effect of transplacentally acquired antibody. None of fourteen acute-phase serum specimens obtained early in the course of culture-positive infections of young infants had detectable antibody to influenza A viral hemagglutinin by the sensitive radioimmunoprecipitation test. Because passively transferred maternal antibody to influenza virus may prevent symptomatic infection in young infants, vaccination of pregnant women could be beneficial.