One-way wife-husband and wife-unrelated donor mixed lymphocyte reactions (MLR) were performed for 17 patients with malignant gestational trophoblastic disease (MGTD) and 17 normal pregnant subjects in the first trimester. The response of patients with MGTD to the husbands' lymphocytes did not differ significantly from the response to donors' lymphocytes when the MLR was performed in pooled human serum. Autologous plasma from patients with MGTD suppressed the MLR to husbands' lymphocytes but not that to donors' lymphocytes. Plasma from pregnant subjects in the first trimester did not suppress the wife-husband MLR. It is postulated that the specific plasma blocking activity may contribute to the failure to reject the trophoblastic tumours.