Hypoimmune responses towards noninherited maternal MHC antigens (NIMA) have been observed on both the humoral and cellular level in humans. The mechanism involved is not known but is thought to involve transfer of material from mother to child. Here we use a mouse model to test this hypothesis. Mice exposed to NIMA either in utero and/or via breast milk showed enhanced survival of maternal skin grafts and also had maternal lymphocytes in their lymph nodes. Survival of the maternal skin graft correlated with the number of maternal T cells present and was not correlated with the number of maternal B cells. We propose that the transferred maternal T cells selectively inactivate host T cells capable of recognizing them.