Leukocytes of dogs (carnivora) and of horses (herbivora) were studied. Dog leukocytes were richer in trypsin than horse leukocytes. Fresh horse leukocytes had little tryptic action, but leukocytes that had stood for a time had strong tryptic action. One gm. of dried horse leukocytes contained 78-164 small units of trypsin, and 1 gm. of dried dog leukocytes 152-333. Leukocytes from a healthy dog had greater tryptic action than from a sick dog. Only a part of the blood cell trypsin was soluble in water or glycerin. Trypsin from blood cells acted toward poisons much as yeast cell enzymes did. Intestinal kinase activated leukocyte trypsin. Some preparations of leukocytes contained much less trypsin than others. There is physiological as well as pathological variation in the trypsin content of the blood. Peptidase was found in leukocytes of both dogs and horses.