Mouse pancreatic proteases were analyzed by one- and two-dimensional electrophoresis. Active proteases that existed in the luminal fluid were separated into at least eight bands in 8% polyacrylamide gel. Pancreatic proteases activated by intestinal extract were separated into at least seven bands. The mobilities of these bands were exactly the same as those of proteases in the luminal fluid except for those of the most cathodal band. Two kinds of trypsin (Try-I group and Try-II) and one kind of chymotrypsin (Chy-I) were determined by specific and nonspecific protease staining. Try-I group and Try-II were derived from different trypsinogens (Try G-I group and Try G-II), whereas Chy-I was derived from a single chymotrypsinogen (Chy G). Although Try G-II was activated by both intestinal extract and by bovine trypsin, Try G-I group activated only by intestinal extract. Intestinal-activating factors were analyzed by two-dimensional electrophoresis. Mouse enterokinase (enteropeptidase EC 3.4.4.8), which can activate bovine trypsinogen, had a slow mobility. In the intestine of the mouse there are several activating factors in addition to enterokinase. Although it is unclear what intestinal-activating factors can activate Chy G, there is a factor that can convert chymotrypsinogen into chymotrypsin directly. These data suggest that intestinal-activating factors play an important role in the activating mechanisms of mouse pancreatic zymogens.