AIMS--To investigate the role of serum and neutrophil tumour necrosis factor alpha (TNF alpha) in patients with viral hepatitis. METHODS--The activities of serum and neutrophil TNF alpha were measured using a bioassay of in vitro cytotoxicity against L929 cells in 57 patients with viral hepatitis and 20 healthy blood donors. RESULTS--Both serum and neutrophil TNF alpha in patients with chronic active hepatitis (CAH) and subacute fulminant hepatitis (SAFH) increased compared with those in normal controls (p < 0.01). No such differences were seen in patients with acute hepatitis. Serum and neutrophil TNF alpha were obviously reduced in patients with CAH and SAFH during convalescence compared with the active period (p < 0.05; p < 0.01). Furthermore, serum TNF alpha was significantly increased in patients with SAFH and complications compared with those without (p < 0.01), and in patients with SAFH who died compared with those who survived (p < 0.01). Neutrophil TNF alpha was significantly higher in patients with SAFH and secondary bacterial infections (p < 0.05). CONCLUSIONS--Production of serum and neutrophil TNF alpha is increased in patients with CAH and SAFH, suggesting that neutrophil TNF alpha causes liver injury in these patients.