To investigate the role of natural killer (NK) cells in the induction and pathogenesis of graft-versus-host (GVH) disease, +/beige (+/bg; normal NK cell activity) and beige/beige (bg/bg; deficient NK cell activity) parental C57BL/6 (B6) lymphoid cells were used to induce GVH reactions in either B6 X C3H/Hej +/bg (+/bgF1) or B6 X C3H/HeJF1 bg/bg (bg/bg F1) hybrid mice. When B6 bg/bg parental lymphoid cells (PLC) were injected into bg/bg F1 mice, early splenomegaly, early severe suppression of the plaque-forming cell (PFC) response to sheep red blood cell (SRBC), and only partial suppression of T cell mitogen responses to concanavalin A (Con A) and phytohemagglutin (PHA) were observed on day 12 after GVH induction. In the same GVH combination, slightly augmented NK cytotoxic activity was induced and no GVH-induced moderate-to-severe pathological alterations in the liver and pancreas were observed. When bg/bg PLC were injected into +/bg F1 mice, early splenomegaly and pronounced immunosuppression of the PFC response to SRBC and partial suppression of Con A and PHA responses were observed on day 12 after GVH induction. In this combination (bg/bg----+/bg F1), significant NK cell activity was induced, but no moderate-to-severe histopathological alterations were observed. In contrast, when B6 +/bg PLC were injected into either +/bg F1 or bg/bg F1 hybrids, early splenomegaly, and severe immunosuppression of both the PFC response to SRBC and the T cell mitogen responses to Con A and PHA were observed by day 12--which persisted until day 30 after GVH induction. Furthermore, high NK cell activity was recorded and moderate-to-severe histopathological alterations appeared in both +/bg F1 and bg/bg F1 recipients. These results show that the bg/bg PLC can induce GVH-associated early splenomegaly and immunosuppression of the PFC response to SRBC in both the bg/bg F1 and +/bg F1 hybrids, but that it failed to induce moderate-to-severe histopathological alterations, even though NK cell activity of host origin was activated during GVH reactions. Conversely, when +/bg donor cells were used to induce the GVH reaction, splenomegaly and immunosuppression, as well as moderate-to-severe histopathological lesions were induced. These results suggest that donor NK cells rather than host NK cells play an active role in GVH-associated tissue damage, which in turn contributes to the long-term suppression.