In this study, we assessed the underlying mechanisms by which proteinases released from activated neutrophils mediate fibronectin degradation. Purified human neutrophils (1 X 10⁶) were incubated for 1 hr with 1 μM PMA in the absence or presence of different prateinase inhibitors in 96-well microtiter plates that were coated with¹²⁵I-labeled fibronectin (FN). PMA-activated neutrophils caused 85% of FN to be degraded (versus5% under control conditions). A selective inhibitor of elastase (L658,758), a monoclonal antibody directed against human neutrophilic elastase, and plasma all reduced the: neutrophil-mediated FN degradation by 60%. A monoclonal antibody directed against the neutrophil adhesion glycoprotein CD 11 /CD 18 increased the antiproteoly tic effect of plasma to 70 % but had no effect on the other anti-elastase agents, suggesting that the subjacent space formed by adherent neutrophils restricted to a small degree plasma derived antiproteinases. Agents that blocked cathepsin G or cathepsin G and elastase completely prevented the proteolysis associated with PMA-stimulated neutrophils, suggesting that the actions of elastase may be dependent on the presence of biologically active cathepsin G.