Our aim was to study protease activity in GCF from inflamed sites with or without tissue destruction. 19 patients with both periodontitis and gingivitis sites and 12 patients having gingivitis alone participated in the study. GCF samples were collected by an intracrevicular washing method. The protease activity was measured as degradation of FITC-conjugated casein. To obtain a semiquantitative estimate of the harvested GCF volume, we measured the transferrin concentration in the wash-fluid. The protease activity was significantly higher in the deep pockets in periodontitis patients than in shallow pockets in the same patients. This difference was still higher when the ratio of protease activity to the amount of transferrin in the sample was plotted. Although protease activity was lower in samples from gingivitis patients than in the deep pockets in periodontitis patients, the difference was not significant. About 90% of the activity could be inhibited by the addition of an excess amount of alpha-1-antitrypsin (A1AT). This study shows that protease activity is higher in inflamed sites with tissue destruction than in inflamed sites without. Most of this activity could be inhibited by A1AT, which suggests that the activity is due to an imbalance between protease and antiprotease rather than to proteases insensitive to A1AT.