The levels of peripheral blood lymphocytes expressing the receptor for transferrin (TSR) on untreated and phytohemagglutinin (PHA)-stimulated cell samples from maternal-neonate pairs were evaluated 4-12 h postpartum. Significantly increased levels of TSR+ cells were observed on fresh, unstimulated neonatal and maternal cells, as compared to control cells from young adult males and females, and the values seemed to correlate with the sex of the neonate and with birth order. The level of TSR+ cells in culture was found to be increased on neonatal cells and decreased on maternal cells.