The mechanisms leading to the presence of fetal lymphocytes in the maternal circulation were studied. HL-A antigens were tested in 85 primiparous mothers and their firstborn children immediately after delivery. In addition, 26 fathers were tested, and antibody followup studies were made in 30 of the women within 1 year after delivery. In lymphocytes from the newborn, the HL-A antigens inherited from the mother were readily demonstrable (only 3 out of 170 antigens caused trouble). In contrast, paternally derived antigens were often difficult to demonstrate in fresh lymphocytes from cord blood. The expression of the paternal antigens could be restored by incubating the lymphocytes overnight under tissue culture conditions. Cell samples from 31 infants were retyped for HL-A after incubation, and in 11 infants 13 previously indistinguishable paternal antigens were detected. The presence of seven more indistinguishable paternal antigens in seven infants was deduced. Lack of phenotypic expression was not associated with any particular HL-A antigen specificity. Hence the masking of paternal antigens was random. Cytotoxic antibodies against paternally derived fetal antigens were found in 10 of the 85 mothers (22%) but they were not cytotoxic to the cord blood lymphocytes unless these had undergone overnight incubation. The masking of paternal HL-A antigens in cells from newborn infants may have contributed to the inducement of immunological tolerance between mother and fetus.