A patient with phenotype R2r and anti-C has a hemolytic transfusion reaction (HTR) with hemoglobinemia and hemoglobinuria which occurred within 2 h of receiving an R1r transfusion. Transient impaired renal function ensued. A patient with phenotype R2R2 and anti-Ce+e had the same experience on day 4 after receiving three R1r and one rr units. 2 other patients, 1 R2r with anti-C who received one R1r unit and the other R2R2 with anti-Ce+e who received two R1r units, showed no clinical evidence of HTR. Both anti-C antibodies were entirely IgG while both anti-Ce+e antibodies initially were predominantly IgM. IgG subclassing was unsuccessful and red blood cell-mononuclear phagocyte assays were normal. These cases occurred from 1979 to 1981.