Animal experimentation has advanced the understanding of cellular events in the afferent and efferent phases of graft-versus-host disease. New immunosuppressive agents such as FK-506, deoxyspergualin, and interleukin-1 receptor antagonist have shown successful prevention and treatment of graft-versus-host disease in rats and mice. Pretransplant and posttransplant immunosuppressive regimens capable of enhancing engraftment of T-cell-depleted marrow have been defined in murine marrow transplantation. Evidence has begun to emerge indicating that engraftment can be enhanced by T cells that do not cause graft-versus-host disease. Finally, investigations have begun to define approaches for enhancing graft-versus-leukemia effects not only in allogeneic marrow transplantation but also in autologous transplantation.