Tryptase-like activity has previously been identified biochemically in gingival homogenates and gingival crevicular fluid (GCF) using substrates linked to the 7-amino-4-trifluoromethyl coumarin (AFC) leaving group. In the present study, activity was demonstrated histochemically in tissue sections with analogous 4-methoxy-2-naphthylamide (MNA) substrates. Z-Ala-Ala-Lys-MNA and D-Val-Leu-Arg-MNA were the most sensitive substrates. Comparison of staining patterns with the MNA substrates and toluidine blue indicated that enzyme activity was localized to mast cell secretory granules. Most stained cells were in the lamina propria, but a few were in the epithelium. The number of stained cells was somewhat greater in inflamed tissue from chronic periodontitis patients than in healthy tissue from controls. However, hardly any staining was seen in inflamed granulomatous tissue. Using high-salt buffer containing heparin, it was possible to extract enzyme activity from tissue sections for biochemical analysis with corresponding AFC substrates. Inhibitors gave similar results in the biochemistry and histochemistry. The inhibitor response and pH profile of the enzyme were the same as that found earlier with gingival homogenates and GCF and were again consistent with mast cell tryptase. The enzyme may have a role in the pathology of chronic periodontitis.