Recent studies on the localization of the serine protease salivary kallikrein have led to the conclusion that it has a ductal localization and to the possibility that it is located there in small "secretory" granules. Until now, the latter inference has been based entirely on circumstantial evidence. In the present study, however, direct evidence from immunolocalization studies at the ultrastructural level establishes the localization of this enzyme in the apical duct granules in the cat submandibular gland. These granules stained only with immune sera to cat submandibular gland kallikrein and were the only subcellular structures that did so. They showed the "studded" appearance characteristic of the electron-dense aggregates of peroxidase-antiperoxidase seen in this type of immune reaction.