The cytotoxicity of plasma from patients with various types of liver disease to rabbit hepatocytes maintained in vitro has been investigated using a microcytotoxicity assay system. Plasma from patients with fulminant hepatic failure and uncomplicated viral hepatitis showed significant cytotoxicity compared to controls. The cytotoxic effect of plasma from patients with fulminant hepatic failure was reduced by charcoal haemoperfusion, or heating and dialysis. The bile acids chenodeoxycholic acid and lithocholic acid were cytotoxic when added to control human plasma. Such toxic factors may be responsible for the delay in liver regeneration often seen in patients with fulminant hepatic failure.