Tryptophan pyrrolase activity of rat liver was markedly inhibited by serum in vitiro, and its inhibition was reversed by hematin. This inhibition by serum was due to the hamatin-binding capacity of serum proteins, espec- ially the serum albumin. Catalase and non-protein substances in the serum were not involved in the inhibition process. Inhibition of tryptophan pyrrolase activity by serum could not be prevented by the substrate, tryptophan in vitro.