Activated polymorphonuclear leukocytes (PMN) release various types of proteases and express them on the cell surface. The proteases play important roles in PMN-mediated events. In the present study, flow cytometric analysis revealed that CD 14 expression on human gingival fibroblasts (HGF) was markedly reduced by PMA-activated PMN in a coculture system. We found that this reduction was caused by secreted proteases from activated PMN. A protease responsible for the reduction was found to be human leukocyte elastase (HLE) by use of various protease inhibitors. Analysis with purified HLE revealed a timeand dose-dependent reduction of CD 14 on HGF, and complete reduction was observed by 680 nM HLE-treatment for 30-60 min. This reduction of CD 14 resulted from direct proteolysis by HLE on the cell surface, because HLE reduced CD 14 on fixed HGF and also on purified cell membranes. As a result of CD 14 proteolysis, IL-8 production by HGF was suppressed when triggered by 10 ng/ml LPS but not by IL-1 α, indicating that HLE inhibited a CD 14 dependent cell activation. These findings suggested that activated PMNs have a potential negative feedback mechanism for HGF function at the inflammatory site, particularly in periodontal tissues.