Abstract: A colorimetric assay system for diagnosing periodontal disease utilizes a chromogenic test substance for measuring proteolytic activity in a specimen such as subgingival plaque which may contain suspected periodontopathogenic bacteria. The chromogenic test substance comprises a peptide substrate which is hydrolyzable by proteolytic enzymes in the plaque specimen to release a chromophore. Detection of a color change in the test substance indicates whether such proteolytic activity is present. In a specific illustrative embodiment, the chromogenic test substance comprises N-benzoyl-DL-arginine-2-naphthylamide (BANA) or benzoyl-DL-arginine-p-nitroanilide (BAPNA). In the case of BANA, for example, the chromophore, .beta.-naphthylamide, is detected by the addition of a color developer, such as fast garnet. The development of a red-orange color is interpreted to indicate the presence of periodontal disease associated with anaerobic periodontopathogens, such as B. gingivalis, T. denticola, and B.