Immune hemolytic anemia due to minor ABO incompatibility between recipient and donor is a well-recognized occurrence in kidney and liver transplantation, In some cases, the responsible antibodies have been shown to be derived from the donor passenger lymphocytes using Gm allotyping. We report a case of acute, transient hemolysis following heart-lung transplantation in which serologic and Gm allotype studies confirmed the etiology of hemolysis.