To investigate the mechanisms of the beneficial effect of blood transfusions (BT) on subsequent kidney transplant survival, we studied the influence of 3 planned BT on lymphocyte reactivity and on lymphocytotoxic antibody (LT) production in previously non-transfused uremic patients. A sustained and non-specific decrease in mixed lymphocyte reactivity (MLR) was observed in approximately 60% of the cases, whereas other patients had only a transient decrease but otherwise normal or even increased responses. Neither the pre-BT degree of immune responsiveness nor the clinical status of the patients had any influence on this phenomenon. Similarly, hepatitis B seroprophylaxy or vaccination did not interfere with this BT effect. Anti-HLA, LT, which are potentially harmful for the transplant, were noted in 13% of cases, while 13% additional cases displayed "cold" anti-B lymphocyte LT which do not have the same prognostic value. In some instances, suppression of cellular reactivity developed concurrently with LT production, which indicates that there is no interaction between cellular and humoral responses induced by BT.