Complement fixing antibody titers to the adenoviruses were determined in 251 newborn infants, using cord blood. Approximately 95% of these were found to have CF titers of 1:16 or over, the majority (75%) were 1:32 or more. Material from the pharyngeal and rectal swabs of these infants on tissue culture studies (542) on HeLa and amnion cells showed no cytopathic effect in oven 96% of these infants. A longitudinal study of 114 of these infants was made at 3 months intervals; 67 have been observed for one year. At 3 months, only 12% still showed CF antibody titers, and these were chiefly at a low level, 1:16. At the subsequent 3-month interval observations, a gradual rise in CF antibodies were found. At one year of age, approximately 37% had titers of 1:32 on over. The findings of this report support the statement that CF antibodies to adenovirus pass the placental barrier. There is a gradual increase in the percentage of infants with positive CF antibodies after 3 months.