Abstract: The activity of tumor necrosis factor (TNF) in both the culture supernatant of pheripheral blood mononuclear cells (PBMC) and the serum was examined in 20 patients of advanced schisto-somiasis at 8th week after pyquiton treatment,and 6 rabbits infected with S.japonicum cercariae at 8th and 16th weeks post-infection and using L929 cell cytotoxic assay.The results showed that the capacity of PBMC to produce TNF in response to LPS from patients with late schistosomiasis was significantly lower than that from normal subjects (P0.01),whereas the activity of TNF in serum was significantly elevated (P0.01).The capacity of PBMC to produce TNF and the TNF activity in serum of infected rabbits showed sharp rise at 8th week post-infection (P0.01),and remained high at 16th week,then returned to normal level 8 weeks after treatment.The persistent rise of TNF in the circulation indicate that TNF might take part in the formation of hepatic fibrosis.