The severity of the graft-versus-host (GVH) reaction, judged by splenomegaly and immunosuppression, was augmented by murine cytomegalovirus (MCMV) infection. Profound GVH-induced immunosuppression was seen in adult unirradiated MCMV-infected F 1, mice even after challenge with extremely low doses of parental spleen cells. Mice receiving MCMV+ GVH challenge died from days 16–21, with interstitial pneumonia being the most prominent pathological lesion. Pulmonary disease was unrelated to levels of viral replication in the lung. These results suggest that in human marrow recipients, cytomegalovirus infection may play a primary role both in provoking or accentuating GVH disease, as well as in the development of interstitial pneumonia.