Summary: GVHR can be induced across the H-2D locus, presumably uncontaminated by any other genetic differences (Ir); it can also be induced across differences that do not have any histocompatibility effect on skin grafts. This is interpreted as evidence for two types of genes in the H-2 complex, H-genes involved in both GVHR and skin graft rejection, and Gvh-genes involved in GVHR but not in skin graft rejection.