Administration of repetitive sublethal doses of either recombinant human tumor necrosis factor (TNF) or recombinant murine γ-interferon (IFN) to non-tumor-bearing (NTB) rats caused a significant decline in food intake and body weight. After 3 days rats became resistant to the anorectic and weight loss effects of TNF but maintained persistent diminished food intake and diminished body weight gain while receiving recombinant murine IFN. Passive immunization against recombinant rat γ-interferon allowed cachectic tumor-bearing (TB) rats to eat more food, have a lesser decline in body weight, live longer, and tolerate larger tumors than similar TB rats given nonspecific control antibody. TB rats treated with an antisera to recombinant murine TNF, which was 100% protective when given to NTB rats 6 h before a lethal endotoxin challenge, did not differ significantly from TB rats treated with control antibody with respect to food intake, body weight, survival, or tumor size. Serum levels of TNF or IFN were not detectable in cachectic tumor-bearing rats. The data indicate that the administration of exogenous IFN can simulate cachexia in NTB rats and that passive immunization against it can partially reverse the cachectic changes associated with cancer and prolong survival. These findings suggest that γ-interferon may be an important mediator of cachexia in this rat tumor model.