When comparing, in a murine model, the kind of graft-versus-host (GVH) disease (GVHD) induced by the donor strain DBA/2 on the one hand and several H-2-congenic resistant B10 donor strains on the other, we found that strain DBA/2 was a universal nonkilling GVH donor for H-2-incompatible nonirradiated F1 hybrid recipients. In this respect, DBA/2 T cells differed from those of the H-2-identical donor strain B10.D2 as well as those of other b10 donor strains. The inability of strain DBA/2 to kill by GVH reaction was not limited to certain H-2 incompatibilities in the F1 recipients, but was nonspecific. The inability to kill is determined by a dominant locus not linked to H-2. DBA/2 T cells were also incapable of inducing the severe suppression of hematocrit values, bone marrow erythropoiesis, thymic cell proliferation, and splenic IgG production in the F1 recipients that was observed after the injection of T cell from the B10 strains. However, DBA/2 T cells, in contrast with those of the B10 donor strains, were vigorous stimulators of IgG production in H-2-incompatible F1 hybrid recipients. Surprisingly, strain DBA/2 as well as the B10 donor strains had good capacity to generate anti-F1 TK cells. Taken together, these findings raise the possibility that lethal GVHD disease is not caused, or not caused exclusively, by donor killer T cells, but by those donor T cells that directly or indirectly induce a suppression of cell proliferation in certain vital organs of the recipient.