We have examined the cellular changes taking place in rat popliteal lymph nodes undergoing a graft-versus-host (GvH) reaction. Examination of immunoperoxidase-stained lymph node sections, using a panel of mouse monoclonal antibodies directed against different rat lymphoid cell subsets, revealed a disorganization of the lymph node architecture with disappearance of the follicles, and an intermingling of T and B cells, so that no distinct T- and B-cell areas were visible any more. Since the GvH nodes showed a preferential accumulation of host B cells over host T cells (particularly over the W 3/25+ T helper cell subset), we also investigated the requirements for host B cell activation. The popliteal lymph node GvH reaction was induced in (PVG X DA)F1 rats by the injection of PVG cells into one foot and by DA cells into the other foot, and then immunoglobulin kappa allotype marked PVG B cells from athymic donors were injected intravenously. The allotype marked B cells proliferated vigorously in response to the DA T cells, but much less in response to the PVG T cells. These results indicate that the massive B-cell activation taking place in GvH reactions may require an alloantigen incompatibility between donor T cells and host B cells, and argue against non-specific mitogenic induction of the B cells.