This program project proposes a coordinated multidisciplinary program of clinical and basic research by established laboratories within Memorial Sloan-Kettering Cancer Center which is aimed at the study and elucidation of the hematopoietic, genetic, immunologic and microbiological features of donor and host which constitute or influence the biology marrow transplantation, as reflected in patients undergoing marrow transplantation for aplastic anemia and leukemia. Research proposals will examine three clinically important problems in marrow transplantation: 1) engraftment and graft rejection, with particular attention to processes affecting reconstitution of hematopoietic and lymphoid function, 2) graft vs. host disease: the hematopoietic and immunological factors contributing to its pathogenes is and affected by its development, 3) infection: host factors contributing to infection by opportunistic pathogens, the effect of invading pathogens on the course of engraftment, and the role of infection in the pathogenes is of GVHD. The clinical marrow transplantation program is the focus of this research effort, and will incorporate therapeutic protocols complementing this the objectives of this research. Thus, the clinical protocols will use modifications of established methods for preparation of the transplant recipient concentrating on development of improved approaches to the prevention and/or treatment of Graft vs. Host disease and infection.