C57BL/6 ob/ob (C57 ob/ob) mice infected persistently with lymphocytic choriomeningitis virus (LCMV) show cachexia as judged by a weight loss of greater than 20%. Virus persists in a subset of macrophages. Because a cachexic state occurs in several chronic debilitating diseases of humans, often accompanied by persistent microbial infections with macrophage/monocytic involvement and tumor necrosis factor (TNF) cachectin production, the role of TNF in the weightloss of ob/ob mice infected persistently with LCMV was investigated. TNF mRNA expression was not increased in peritoneal cells from such persistently-infected mice, nor did their serum levels of TNF rise above those in uninfected litter-mates. Furthermore, in vitro LCMV infection of adherent peritoneal cells from these C57 ob/ob mice did not enhance TNF mRNA or protein expression. Therefore, the cachexia-like weight loss observed in C57 ob/ob mice during a persistent LCMV infection is apparently not associated with a measurable increase in TNF.