Abstract INTRODUCTION THERE are numerous reports in the literature of the presence of an antitryptic (antienzyme) agent in the living tissues and blood sera of animals. However, no report of antitryptic action by extracts of the large roundworm, Ascaridia galli, has been found. In view of the fact that Ascaradia galli lives in the lumen of the small intestine of the bird, where it is continuously in contact with tryptic enzymes, the question arises as to how it is able to resist the proteolytic action of these enzymes. REVIEW OF LITERATURE The nature of antiferments has been studied in considerable detail but there is no agreement as to their mode of action. They are destroyed by heat at about 70°C. by the action of dilute acids, and by prolonged standing. Burge (1915) suggests that the protection of parasites and perhaps the normal alimentary epithelium against proteolytic enzymes depends upon the active . . .