The presence and localization of PMN/neutrophil elastase and its endogenous inhibitor alpha 1-proteinase inhibitor (alpha 1 PI) were studied immunohistochemically in gingival tissue specimens collected from 9 adult periodontitis (AP) patients during flap surgery after the initial phase of periodontal therapy, and from 6 healthy controls with clinically-healthy periodontium upon surgical extraction of impacted third molars. In order to evaluate how periodontal tissue destructive events are reflected in gingival crevicular fluid (GCF), GCF samples were collected from the AP patients before any periodontal treatment and prior to flap surgery, from 5 localized juvenile periodontitis (LJP) patients, and from the controls. Elastase activity in the GCF was measured with the SAAVNA-assay and the molecular forms and amount of alpha 1PI by Western- and dot-blotting. Immunohistochemical staining for PMN elastase was strongly positive in the connective tissue, but not in the epithelium, of the AP patients' gingival tissue specimens. In the healthy gingival tissue specimens only a few elastase-positive cells were present. Both in AP and in control gingival specimens, alpha 1PI was detected in the connective tissue and in the keratinized layer of the epithelium, however, its amount was markedly lower in the control specimens. Elevated levels of alpha 1PI and PMN elastase were detected in the GCF of all periodontitis patients when compared to controls.(ABSTRACT TRUNCATED AT 250 WORDS)