It has been reported that a dramatic decrease in the number of thymocytes (thymic atrophy) in mice suffering from acute graft-versus-host disease (GVHD) is ascribed to glucocorticoids. In this study, we examined the possibility that cellular immune responses may thus be involved in thymic atrophy. In contrast to chronic GVHD mice, acute GVHD (C57BL/6 X DBA/2) F1 (BDF1) hybrid mice, which were injected intravenously with both spleen and lymph node cells from C57BL/6 mice, showed a dramatic decrease in the number of CD4 CD8 double-positive thymocytes 2 or 3 weeks after the induction of GVHD. A flow cytometric analysis revealed the donor-derived T cells with either CD4 or CD8 molecules to infiltrate the thymus of the mice undergoing acute GVHD for 10 days. In a cytolytic assay, such thymus-containing cells exhibited a cytolytic activity specific to the host cells. In addition, anti-H-2d cytolytic T cells showed a high level of cytolytic activity against BDF1 (H-2bXd) thymocytes, whereas they also showed a low level of cytolytic activity against C57BL/6 (H-2b) thymocytes, thus suggesting that the thymus-infiltrating donor-derived T cells killed the host thymocytes through both anti-H-2d-specific and non-specific mechanisms. Interestingly, a flow cytometric analysis revealed both the percentage and the absolute cell number of host-derived NK1.1+ CD3+ cells to increase in the thymus of mice suffering from acute GVHD for 10 days. In addition, they also showed the cytolytic activity against YAC-1 cells and the mRNA expression of interleukin-12 (IL-12) in the thymus to be also significantly augmented on day 7 after the induction of acute GVHD. Collectively, our results indicate that the cellular immune responses such as donor cytotoxic T lymphocytes and host NK1.1+ T cells are therefore involved in the thymic atrophy of mice suffering from acute GVHD.