Expression of OX-40 on PBL during GVHD. (Lewis × Buffalo)F1 rats were injected with either syngeneic F1 (left panel) or allogeneic Buffalo (right panel) BM and LN cells, as described in the text. The percentage of PBL staining with OX-40:FITC (absissa) and CD4:PE (ordinate) was determined 10 days posttransplantation as described in the Materials and Methods.