An apparent inhibition of induction of mouse hepatic tryptophan oxygenase by endotoxin has been reported previously, as evidenced by low catalytic activity. This could be due either to decreased tryptophan oxygenase levels or to inactivation of existing enzyme molecules. To resolve this question, the enzyme was quantitated immunologically in control and endotoxin-poisoned mice. Tryptophan oxygenase was purified and used as an antigen to prepare antienzyme antibodies. The antiserum was shown to be monospecific by immuno-electrophoresis. Addition of the antiserum to high-speed supernatant fluids of liver homogenates of control or endotoxin-poisoned mice resulted in precipitation of the enzyme. Radial immunodiffusion assays revealed that there was less enzyme in livers of mice that received 1 mean lethal dose of endotoxin. It was concluded that endotoxin interfered with the synthetic process that results in enhanced levels of tryptophan oxygenase.