Observations are reported on a number of newborn and premature infants naturally infected with adenovirus types 1, 2, and 3, and ECHO virus types 9, 18, and 20 during their first few weeks of life. Serial serologic studies revealed that these babies generally responded promptly with antibody formation and that this was independent of age and birth weight, but did vary with the particular type of virus involved. The presence of homologous passively acquired antibody did not appear to interfere with the formation of active immune bodies. However, the response to the various viruses differed sufficiently that it was not possible to arrive at general conclusions on the basis of information derived from any single agent.