The process by which cell-mediated immunity is reduced in pregnancy is uncertain. Reports on the response of newborn lymphocytes to phytohaemagglutinin (PHA) are conflicting. We have studied the response to PHA of paired maternal and newborn lymphocytes obtained initially at the time of delivery and subsequently 7 days later. In order to confirm the presence of an inhibitory plasma factor, we compared the PHA responses of maternal and newborn lymphocytes cultured in the presence of their own

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... sma with that after washing and resuspension in fetal bovine serum. The lymphocyte mitotic response was assessed by measuring the rate of 1251-5-iodo-21-deoxyuridine uptake into DNA. Our findings suggest that the reduced lymphocyte response to PHA in pregnancy is due to a plasma inhibitory factor. However, its action was not demonstrable in maternal blood taken 7 days after delivery. DNA synthesis in unstimulated cultures from newborn infants at birth and at 7 days was significantly greater than that of adult controls. With PHA stimulation, the response of cord blood lymphocytes paralleled that of adult controls when cultured in their own plasma, but washed newborn cells showed significantly greater responses. Therefore plasma suppression similar to that observed in mothers also appears to affect infants at birth. This inhibition was not demonstrable in blood taken from infants 7 days old. The nature and source of this inhibitory factor is open to speculation and further studies are required to clarify its role in the survival of the fetal allograft in human pregnancy. Serum IgG levels in feto-fetal transfusion syn-