Plasma and platelet benzylamine oxidase activity was tested before and after portacaval shunt in patients with histologically-confirmed cirrhosis of the liver. The level of plasma benzylamine oxidase activity was always higher in cirrhotic patients than in control subjects and was unchanged by the surgical operation. Platelet benzylamine oxidase activity was lower in cirrhotic than in control patients before the surgical operation, and higher afterwards but it returned to the original value at 80 days after surgery.