Little is known about the etiology of the graft-versus-host disease (GVHD) occuring after transplantation of lymphoid cells incompatible for minor histocompatibility antigens (mHAg). Here, the potential role of host endogenous mouse mammary tumor virus (Mtv)-encoded superantigens (SAg) in the development of lethal GVHD was investigated. In a combination of H-2d compatible mice, the presence of Mtv-7 and, to a lesser extent, of Mtv-1, -6, -13 in the host genome, highly increases the rate and severity of GVHD. Kinetic analyses of TCR V beta gene expression in recipient mice consistently indicate a dramatic but transient infiltration of GVHD target organs by Mtv-SAg-specific T cells. This suggests that SAg encoded by endogenous Mtv, by activating large T cell subpopulations, would help the response to mHAg and thus play a critical role in the initiation or aggravation of GVHD.