1. Fresh and inactivated animal serum under proper conditions will bind pepsin quantitatively in weak acid solution and will prevent it from digesting proteid even after the addition of free hydrochloric acid in excess. 2. This binding and inactivation of pepsin cannot be considered as due to a specific anti-pepsin. 3. The phenomenon has been named pepsin deviation in analogy with the deviation described for other ferments, notably trypsin. 4. The ability of animal serum to deviate pepsin has been responsible for most, if not all, of the published accounts of anti-pepsin. 5. By the use of a technique elaborated to control pepsin deviation, it has been found impossible to demonstrate normal anti-pepsin in the blood serum of the dog, cat, guinea pig, beef, horse, rabbit, and of man.