The present status of organ transplantation in man is reviewed. Immune rejection of the graft (the homograft reaction) remains a central problem in transplantation immunology. Efforts to control the homograft reaction include tissue typing to identify the significant transplantation antigens as well as suppression of immune reactivity. A new approach to specific immune suppression is the use of soluble transplantation antigens to induce specific tolerance. Human and mouse transplantation antigens have been prepared in soluble forms and are being characterized chemically and biologically. In bone marrow transplantation the immune reaction of graft against the host—which does not occur in most organ transplants—is an area of special concern. On the other hand, transplantation of bone marrow and related lymphoid tissue does offer opportunities for supplying immune competence that may be useful in treatment of cancer.