Lymphocytes from human newborns inhibit division of their mothers' lymphocytes. Three days after we cultured equal numbers of cells from a mother and her baby in the presence of PHA, mitosis of the mother's lymphocytes was suppressed 13-fold compared to that of the baby's lymphocytes. At the end of 3 days the number of baby's lymphocytes were doubled those of the mother's. The survival rates and mean mitotic indexes of both pairs of cell were roughly equivalent (mean +/- S.E: baby 2.4 +/- 0.8; mother 2.6 +/- 0.7), indicating that the lack of dividing lymphocytes from the mother was caused by inhibited division of the mother's lymphocytes, no enhanced growth of the newborn's cells. The cell population in newborns that is responsible for the inhibition effect resides in the T cell-enriched population. Lymphocytes from one newborn were not able to inhibit division of lymphocytes from another newborn, suggesting that lymphocytes from newborns could continue to divide despite their inhibitory effect. Other experiments showed that actively dividing fetal fibroblasts, amnion cells from the newborn, and continuous T lymphoblastoid cell lines were unable to inhibit mitosis of lymphocytes of the mother.