The main neutral protease of rat skin is a mast cell enzyme. Immunohistochemical localization of the enzyme in rat skin with the peroxidase-antiperoxidase (PAP) complex method.

The highly sensitive PAP immunoperoxidase method was used to localize the main neutral protease of rat skin. The use of the neutral detergent, Triton X-100, in the reagent and washing solutions was observed to effectively decrease the nonspecific staining. The specific staining was localized to the mast cell granules.