Two zymogens, designated prokallikreins A and B, were isolated from homogenates of rat pancreatic tissue. The two forms of prokallikrein were found to be very similar in size and charge properties. They gave rise to very similar kallikreins on activation with exogenous trypsin. Differences in carbohydrate content of the two zymogens were probably responsible for differences seen in their behaviour on ion-exchange chromatography and immunoelectrophoresis. In contrast, only one form of prokallikrein was isolated from rat pancreatic juice. It showed almost identical behaviour on ion-exchange chromatography and identical mobility on electrophoresis to prokallikrein A. Thus it can be tentatively suggested that it is prokallikrein A which is secreted into the pancreatic juice and represents the physiologically important zymogen.