An examination of fish digestive organs showed that only the pyloric caeca and intestine showed a sufficiently high yield and a sufficiently high activity to make their commercial use feasible. The intestine had approximately one-quarter the activity of the caeca. Mackerel caeca showed the highest activity and yield of any organ in four species of fish examined. No activating effect parallel to that of mammalian enterokinase on trypsinogen was demonstrated. By comparing the hydrolysis products of casein by pyloric caeca, intestinal mucosa, and hog pancreas, evidence was adduced showing that the enzymic constituents of these preparations are similar.