The blood level of tumor necrosis factor (TNF) was measured by the method of TNF cytotoxic activity in 20 patients with abdominal infection, 10 patients with gastrointestinal cancer and 30 normal subjects. The results showed that the blood level of TNF in patients with abdominal infection was significantly higher than that in normal subjects and in patients with gastrointestinal cancer. The blood level of TNF in serious abdominal infection was remarkably higher than that in moderate abdominal infection. The blood level of TNF in both group of patients was obviously declined after the infection had been controled. The blood level of TNF in the patients died from abdominal infection was constant during treatment. Therefore, the blood level of TNF is an important index of abdominal infection and has important clinical value in evaluating the seriousness of abdominal infection and predicting the development of infectious disease.