After hematopoietic stem cell transplantation, delayed immune hemolysis may occur when donor-derived B lymphocytes carried with the graft produce immune antibodies against the recipient's incompatible red cells. We report the occurrence of this syndrome in the context of minor blood group incompatibility between donor and recipient after peripheral blood stem cell (PBSC) transplantation. On day 12 post-transplant there was abrupt onset of hemolysis necessitating supportive treatment with hydration and transfusions. Because, as compared to bone marrow, PBSC grafts are enriched with lymphocytes, more frequent and intense delayed immune hemolysis may be anticipated when using PBSC. This complication is described most often when cyclosporine alone is used for immunosuppression following the graft. The addition of methotrexate, which with CyA forms the classic regimen for the prevention of graft-vs-host disease, may diminish the frequence and severity of this adverse reaction.