1. An equation is derived for the calculation of a constant which, experimental results indicate, may be a more reliable index of the antiproteolytic activity of serum than those equations hitherto used. 2. (a) Intramuscular administration of trypsin resulted in a slow rise in the antiproteolytic activity of the serum, followed by a lesser decline. (b) Intravenous administration resulted in no appreciable variation. (c) Oral administration resulted in a rapid rise, which was sustained during the period of administration. (d) Intramuscular, intravenous, or oral administration of denatured trypsin resulted in no appreciable variation. (e) The extent of the local necrosis following subcutaneous injection of trypsin varied inversely with the antiproteolytic activity of the serum. 3. The experimental results indicate that the products of protein hydrolysis in the intestine and parenterally are an important factor in the antiproteolytic activity of the serum. They also indicate that antibodies to trypsin are not an important factor in the antiproteolytic activity of the serum.