Abstract Accelerated periodontal tissue destruction in patients with labile insulin-dependent diabetes mellitus (DM) and localized juvenile periodontitis (LJP) has been suggested to be related to functional abnormalities of neutrophils. We have recently found that collagenase in gingival crevicular fluid (GCF) of adult periodontitis patients is primarily derived from neutrophils and that neutrophil collagenase activity is more sensitive to inhibition by tetracyclines than collagenase produced by fibroblasts. This study is to characterize the cellular sources, activation and inhibition of collagenase in GCF of DM patients and to compare it with collagenase in LJP GCF. We found differences which may have therapeutic implications. Specific doxycycline inhibition tests revealed that GCF collagenase in DM is derived from neutrophils, whereas the enzyme in LJP originates primarily from fibroblasts. Oxidant, sodium hypochlorite activated efficiently GCF collagenase of DM but not LJP patients. In contrast, plasmin activated LJP GCF collagenase but not that of DM patients. In GCF of DM patients 50–60% of collagenase existed in an active form, whereas in LJP GCF, the enzyme was almost completely in a latent form. The results suggest that collagenase in GCF of periodontitis patients with labile DM is primarily derived from neutrophils and that tetracycline therapy may be an effective adjunct in treatment aimed at controlling the periodontal breakdown in these patients. On the other hand, in LJP the anti-collagenase property of tetracyclines may be less important for control of periodontal tissue destruction because of the tetracycline-resistance of fibroblast collagenase.