T cell immune function in 20 newborn infants was investigated. Previous studies showing increased spontaneous transformation and higher ³H-thymidine incorporation at lower PHA concentration in newborn infants were confirmed. A net increase in the number of active E rosette-forming lymphocytes and a slight decrease in the percentage of total E rosette-forming cells was also found. Our results suggest the presence of a subpopulation of activated T lymphocytes in the peripheral blood of newborns during the first days of life. Possible mechanisms of this activation are discussed.