In a retrospective study involving 24 transplantation centres and 858 cadaveric kidney recipients, a number of transfusion factors affecting transplant survival were identified. The best results were observed with the most intensive transfusion schedules including at least one transfusion per month. The optimal number of transfusions varied from 6 to 20. However, it was impossible to determine whether a minimal interval was required between the last perfusion and transplantation, or whether the effect of each transfusion was limited in time. Qualitatively, it appeared that whole blood and packed red cells gave better results than leucocyte-deprived blood. Moreover, fresh blood taken less than 3 days before the transfusion clearly proved more effective than blood stored for more than 5 days. All this suggests that live leucocytes and platelets may be important factors. The mechanism by which blood transfusions improve the outcome of kidney transplants remains unknown.