Hyaluronidase activity has been studied in gingival crevicular material from clinically healthy and chronically inflamed gingivae, and in neutrophil polymorphonuclear leukocytes (neutrophils) from peritoneal exudates. Activity was demonstrated by determining the release of N‐acetyl‐glucosamine from purified hyaluronic acid. Hyaluronidase activity was found in gingival crevicular material within the pH range. 3‐7. with pH optima at 3.7 and 6.0. Material from inflamed gingivae showed greater enzyme activity than that from healthy gingivae. Hyaluronidase activity was present in homogenates of neutrophils from peritoneal exudates with a pH optimum of 3.7. Little enzyme activity was found above pH 5.0. This suggests that the hyaluronidase activity at pH 3.7 in gingival crevicular material might originate from neutrophils in the gingival cr