Bone marrow transplantation is a worldwide activity involving more than 250 transplant centers in more than 40 countries. The ultimate success of the procedure depends in large measure on supportive care of the transplant recipient. A major component of this supportive care is the prevention of opportunistic infections. This article reviews the nature of immunoglobulin class and subclass deficiency after marrow transplantation and the role of intravenous immune globulin in preventing infections and associated complications.