The trophoblast membrane of the human placenta constitutes the effective interface between blood borne cells of the maternal immune system and foetal tissue. Therefore antigens expressed on this membrane are ideally located to induce an immune response. We have previously described a trophoblast antigen TA which induces a humoral response. In this study the leucocyte migration inhibition assay was employed to assess the cellular response of pregnant women to TA. Leucocytes from primigravidae and multigravidae responded to TA while controls did not. Addition of autologous pregnancy sera abrogated responses to TA but did not affect responses to BCG. We propose that pregnancy sera contains a blocking factor which contributes to the survival of the foetal allograft by suppressing a potentially hostile maternal immune response directed against TA expressed on the trophoblast membrane.