Recent clinical reports of nonresponsiveness to noninherited maternal human leukocyte antigens have led to speculation that humans may acquire tolerance of noninherited maternal antigens through exposure to maternal cells neonatally or in utero. To test this hypothesis, we measured the responsiveness of normal subjects to their noninherited maternal and paternal antigens using cell-mediated lympholysis assays and mixed leukocyte reactions. All individuals exhibited cell-mediated lympholysis and mixed leukocyte reaction responses to the maternal cells that were comparable to those to the paternal cells. Limiting dilution analyses revealed significant cytotoxic T-lymphocyte precursor frequencies to both sets of parental antigens. To exclude the possibility that tolerance of individual noninherited maternal antigens was masked by the response to other antigens expressed on the same target cell, we raised cytotoxic T lymphocytes to the maternal cells and then tested for reactivity to a panel of targets that expressed single noninherited maternal HLA antigens. In all cases, each noninherited maternal antigen expressed on the maternal cells elicited a significant cell-mediated lympholysis response. An analysis of clinical data showed that pretransplant mixed lymphocyte reactions to maternal cells are not significantly lower than those to paternal cells. These data suggest that the reported B-cell tolerance of noninherited maternal antigens is not mediated by clonal deletion of T cells induced by exposure to the maternal cells neonatally or in utero.