Spleen organ cultures were prepared from lethally irradiated mice that had been injected with allogeneic bone marrow, with or without lymph node cells. These cultures were used in the analysis of graft-versus- host reactions (GVHR) in vitro. It was found that donor-derived T lymphocytes, which had been grown in an allogeneic spleen, prevented hemopoietic progenitor cells (colony-forming cells, CFC) of the recipient from differentiating in agar cultures. Furthermore, these lymphocytes were found to inhibit growth and differentiation of CFCs of the same donor. This graft-antigraft reactivity was not abolished by monoclonal anti-Thy-1 plus complement (C'), in contrast to the conventional GVHR. This indicates that the graft-versus-graft reaction depends on “de novo” generation of mature T lymphocyte effector cells from transplanted graft precursors.