One-way wife-husband and wife-donor mixed lymphocyte reactions (MLR) were performed in ten patients with hydatidiform moles and ten age- and maturity-matched pregnant controls. The MLR response in patients with hydatidiform moles was similar to that of normal pregnant subjects and the maternal plasma did not affect the response. In the two patients who subsequently required chemotherapy because of persistent elevation of serum human chorionic gonadotrophin, the maternal plasma taken just before chemotherapy suppressed the maternal MLR response to paternal lymphocytes but not that to unrelated donors' lymphocytes.