The increase in the basal level of activity of rat liver tryptophan oxygenase caused by hydrocortisone disappears if the animal is previously treated with an intraperitoneal injection of human amniotic fluid. The effect of amniotic fluid on the enzyme induction is parallel to the disappearance of the increase in the incorporation of 14-C-L-leucine, which is caused by hydrocortisone in the TCA precipitate of the soluble liver fraction. Experiments using Sephadex G-25 and dialysis fractions show that the inhibition is mostly due to a rather large size molecule.