In a 12-year-old patient with blood group O, who had received a partial liver graft 10 years ago from his father with blood group A, the levels of anti-A-specific antibodies (Abs) were persistently undetectable after the transplantation, while the levels of anti-B-specific Abs gradually increased and attained a plateau. Peripheral blood mononuclear cells (PBMCs) from this patient were engrafted into NOD/SCID mouse in order to investigate the immune response to donor-type blood group antigens. Even after sensitization with blood group-A erythrocytes, no anti-A Abs were detected in the serum samples of the mouse that received PBMCs from the blood group-O recipient of group-A liver allograft, however, immunoglobulins specific for antigens other than the A antigens were produced. Thus, we provide a possible evidence of immune tolerance to blood group antigens in this ABO-incompatible pediatric liver transplantation.