Impaired immunity is a fundamental obstacle to successful allogeneic hematopoietic cell transplantation. Mature graft T cells are thought to provide protection from infections early after transplantation, but can cause life-threatening graft-vs.-host disease. Human CMV is a major pathogen after transplantation. We studied reactivity against the mouse homologue, murine CMV (MCMV), in lethally irradiated mice given allogeneic purified hematopoietic stem cells (HSCs) or HSCs supplemented with T cells or T-cell subsets. Unexpectedly, recipients of purified HSCs mounted superior antiviral responses compared with recipients of HSC plus unselected bulk T cells. Furthermore, supplementation of purified HSC grafts with CD8(+) memory or MCMV-specific T cells resulted in enhanced antiviral reactivity. Posttransplantation lymphopenia promoted massive expansion of MCMV-specific T cells when no competing donor T cells were present. In recipients of pure HSCs, naive and memory T cells and innate lymphoid cell populations developed. In contrast, the lymphoid pool in recipients of bulk T cells was dominated by effector memory cells. These studies show that pure HSC transplantations allow superior protective immunity against a viral pathogen compared with unselected mature T cells. This reductionist transplant model reveals the impact of graft composition on regeneration of host, newly generated, and mature transferred T cells, and underscores the deleterious effects of bulk donor T cells. Our findings lead us to conclude that grafts composed of purified HSCs provide an optimal platform for in vivo expansion of selected antigen-specific cells while allowing the reconstitution of a naive T-cell pool