Transmigrating neutrophils secrete a 92 kDa gelatinase (MMP-9) in order to degrade type IV endothelial basement membrane collagen. A model system for neutrophil adhesion combining a short pre-adhesion time (30 min) in plastic or endothelium-coated wells, medium removal and addition of soluble stimuli (fMLP, TNF alpha), enabled us to induce the release of a basal level of gelatinase activity (> 12% total cell content) from tertiary granules, while the release of vitamin B12 binding protein from specific granules was limited to 4% total cell content. Neutrophil gelatinase activity in unfractionated supernatants from endothelium-coated wells was significantly reduced (P < 0.01) compared to levels obtained on plastic supports, even after TNF alpha treatment or when cell populations were physically separated by trans-well inserts. In contrast, gelatin zymograms of supernatants from plastic and endothelium-coated wells remained similar. These findings suggest that MMP-9 is equally secreted but differentially inhibited by the tissue inhibitor of metalloproteinase-1 originating from the neutrophils. MMP-9 RT-PCR from neutrophils, assessed after up to one hour adhesion on plastic, yielded a single 270 bp fragment which was almost undetectable in the endothelial RT-PCR counterpart, whereas the TIMP-1 PCR product was apparent in both cell types. Furthermore, neutrophil adhesion on endothelial cells and TNF alpha activation for one hour induced the disappearance of MMP-9 cDNA without changes in TIMP-1 and beta-actin PCR products. These results suggest the existence of a dual down-regulation during neutrophil-endothelial interaction, both at the level of secreted MMP-9 activity and of MMP-9 gene transcription.