We have recently demonstrated that high-dose IL-2 administered for a short period (2.5 days) beginning on the day of bone marrow transplantation mediates a marked protective effect against GVHD in mice, while preserving the ability to achieve alloengraftment (1). This protective effect is augmented by administration of T cell-depleted (TCD) syngeneic marrow, and is dependent upon early administration of IL-2 (1). The graft-vs-tumor effect against the EL4 leukemia/lymphoma is not diminished in animals protected from GVHD by IL-2 (2). In an attempt to determine whether or not IL-2-activated host-type cells might be responsible for GVHD protection, we have now performed adoptive transfer studies. The results failed to provide evidence that treatment of lethally irradiated mice with IL-2 activates protective host-derived or syngeneic marrow-derived cell populations which can be adoptively transferred to lethally irradiated secondary recipients receiving allogeneic GVHD-producing inocula. Likewise, treatment of lethally irradiated mice with a complete 2.5-day course of IL-2 prior to administration of allogeneic inocula did not lead to GVHD protection. These results suggest that either IL-2 directly inhibits the GVH reactivity of allogeneic GVH-reactive cells, or that GVH reactivity is attenuated by IL-2 during the period of interaction of donor- and host-type cells.