We studied the effect of depleting NK1.1+ cells from an allograft of lymph node and spleen cells on the outcome of GVH disease in the parent----F1-hybrid combination C57BL/6----(C57BL/6xDBA/2)F1. Four treatment groups were established: group I mice were transplanted with an unmodified graft from normal donors; group II mice were transplanted with an NK1.1-depleted graft that had been harvested from normal donors; group III mice received grafts from donors that had been injected with Poly I:C (100 micrograms i.p.) 18 hr prior to harvesting (These grafts were incubated with complement, but no antibody.); group IV mice were transplanted with depleted grafts harvested from donors that had received Poly I:C. Recipients in each group were monitored for splenomegaly, mitogen responsiveness, NK and CTL activity, histopathology, weight loss, and mortality. Results showed that recipients in all four groups developed splenomegaly and unresponsiveness to LPS, PHA, and Con A by day 12. Augmented host-derived NK activity and graft-derived antihost CTL activity was also demonstrated. Group IV showed little or no weight loss, minimal histopathological changes and a marked reduction in mortality. Recipients in all other groups developed features characteristic of GVH disease and exhibited a steady decline in body weight beginning by day 12. Mortality generally began on day 18 and reached 75-90% by day 60. We postulate that anti-NK1.1 depletes cells from the graft are intimately connected with the effector mechanism in acute GVH disease.