Bone marrow transplantation is emerging as a viable therapeutic approach to a number of diseases that are usually or uniformly fatal. We review here recent experiences in bone marrow transplantation in man at UCLA and in various other institutions throughout the world. We examine marrow transplantation in immunodeficiency diseases, acute leukemia, and aplastic anemia and consider the problems of infection in the transplant recipients. The applications of tissue typing to marrow transplantation and immunologic manipulations, which may influence engraftment and graft-versus-host disease, are also reported.