Epstein–Barr virus (EBV) is the causative agent of infectious mononucleosis (IM). In addition, latent infections with EBV are associated with nasopharyngeal carcinoma (NPC) and Burkitt's Lymphoma (BL). Antibodies to several EBV-encoded early antigens (EA) are often observed in patients with NPC and BL, however, the role of EBV-encoded proteins in the etiology of these and other EBV-associated diseases is not completely understood. The EA complex encodes for at least six different viral enzymes including deoxyuridine triphosphate nucleotidohydrolase (dUTPase). dUTPase has recently been shown to modulate activation of human peripheral blood mononuclear cells in vitro (unpublished data). Therefore, these studies were designed to test whether dUTPase would modulate immune function in an in vivo model. Mice were injected with purified EBV dUTPase, and baseline immune function and sickness behaviors were measured. EBV dUTPase treatment inhibited replication of mitogen-stimulated lymphocytes obtained from treated mice. These lymphocytes were also less able to synthesize interferon-γ after re-stimulation. In addition, treatment with dUTPase induced sickness behaviors. For example, as compared to control animals, dUTPase-treated animals lost body mass, had elevated body temperature, and displayed diminished locomotor activity. These data suggest that individual viral proteins may play a role in the pathophysiology of EBV associated disease. J. Med. Virol. 74:442–448, 2004. © 2004 Wiley-Liss, Inc.