In 1973 we reported significantly superior survival of kidneys transplanted to blood group O recipients compared with recipients of those from blood groups A, B, and AB taken together. In this extended series, the difference between these categories was less prominent and no longer significant. In the present study, blood transfusion significantly improved the survival of kidney grafts in patients of blood group O, but not of combined A, B. and AB groups. The difference between the graft protecting effect of transfusion in group O and combined groups A, B, and AB recipients was also significant. This suggests that the improvement in subsequent graft survival after transfusion is either confined to blood group O recipients, or is much stronger in them than in recipients of other groups. Our previous policy of restriction of blood transfusion is seen as one of the causes of the reduced superiority of group O over other groups in this extended series in comparison with our 1973 series. It seems that transfusion of group O recipients can markedly improve the prognosis of a subsequent first kidney graft.