The effect of planned blood transfusion (BT) on lymphocyte reactivity in previously non-transfused uraemic patients has been investigated. A sustained and non-specific decrease in MLR was observed in approximately 60 per cent of the cases. Other patients had only a transient decrease, normal or increased response. Lymphocyte suspensions whose proliferation was reduced after BT suppressed the response of autologous cells taken before BT. Neither pre-BT degree of immune responsiveness nor clinical status of the patients had any influence on this phenomenon.