Monocyte-depleted lymphocyte suspensions from the peripheral blood of 36 healthy newborns and 23 adults were compared as to percent of cells forming rosettes with sheep erythrocytes (T lymphocytes) and proliferative responses after in vitro culture with various doses of Phytohemagglutinin, Concanavalin A and Pokeweed mitogen. No significant difference was found between the proportions of T lymphocytes of newborns and adults. Mitogen responsiveness of the newborn lymphocytes showed dramatic between-subject variability completely unrelated to the proportion of T lymphocytes. It is suggested that during neonatal life there is massive variable recirculation of T lymphocytes subsets (and/or stages of maturation) characterized by differential degrees of responsiveness to the mitogens tested.