Background: Periodontitis is characterized by extensive destruction of the gingival tissues and associated supporting structures of the teeth. Although the pathogenesis of the various forms of this disease is not completely understood, host‐derived proteases are believed to have an important role. In this study, we analyzed human tissue samples from chronic adult periodontitis patients to assess the levels of specific proteases and determine the effect of pH and tetracyclines on their activity. Methods: Gingival tissue samples were obtained from patients with chronic adult periodontitis (probing depths ranged from 5 to 9 mm) and periodontally healthy controls. Tissue extracts were prepared and analyzed for protease activity by zymography and Western blotting. Results: Maximal protease activity from clinically normal and diseased tissues was observed at pH 8. Latent matrix metalloproteinase (MMP)‐9 and MMP‐2 were expressed in all samples examined, while active MMP‐2 was detected only in tissues obtained from patients with clinical disease. The MMP activities were differentially inhibited by derivatives of tetracycline. At pH 6, a protease with a mass of approximately 40 kDa was observed in diseased samples. The enzymatic activity was inhibited by phenylmethylsulfonyl fluoride, suggesting it is a serine protease. Conclusions: The results of the current study substantiate the proposed role of host‐derived proteases in the pathogenesis of chronic adult periodontitis. Specifically, they indicate that activated MMP‐2 and a 40 kDa serine protease are involved in tissue destruction associated with this form of periodontal disease and also suggest that tissue pH influences protease activity in situ. J Periodontol 2000;71:353‐360.