Antibodies specific for recipient red cell antigens have been demonstrated in patients who receive organs from ABO unmatched donors. In some cases, severe but self-limited hemolysis has been associated with the development of posttransplantation antibodies. We report a case in which an A2 recipient of a kidney from a group O donor formed anti-A1 antibody which reacted with homologous red cells. The patient developed a delayed hemolytic transfusion reaction. A radiochromium survival study revealed decreased survival of A1 red cells. A broader specificity of the antibody was not shown during follow-up. The anti-A1 was not detected in a sample tested 6 months after the initial study. It is probable that the anti-A1 noted in this case was formed by passenger lymphocytes. A2 patients who develop anti-A1 after renal transplantation should be transfused with red cells negative for A1.