A patient with blood group type O accidentally received a blood group B cardiac allograft. He died almost 5 years after transplantation because of transplant vasculopathy. We monitored the expression of ABO-type antigens on the vascular endothelial cells of the cardiac allograft for 44 months, using monoclonal antibodies for immunohistology. The antigenic profile of the graft endothelial cells changed progressively from B to O. This change was first detectable 14 months after transplantation and it was most prominent at the end of the observation period. Thus, we have shown that the allograft blood type of the endothelium of this patient's heart began to change from type B to his own, 1 year after transplantation. By 44 months post-transplantation, it had changed to O type.