Application of leukotriene B4 (LTB4) to hamster cheek pouches induces neutrophil-dependent vascular leakage of macromolecules as well as leukocyte intravascular adherence and emigration. The effect of inhibitors of neutrophil elastase on these reactions was studied with intravital microscopy. Anesthetized hamsters were pretreated with the elastase inhibitors L 658,758, Eglin C, or dextran sulfate, and LTB4 (10 nM) was superfused over the cheek pouches. Neither Eglin C nor L 658,758 had any effect on the resulting vascular leakage of a macromolecular marker; in contrast, dextran sulfate suppressed this leakage by 85%. None of the compounds affected LTB4-induced leukocyte adherence or neutrophil diapedesis. The inhibitors were able to inhibit both hamster and human neutrophil elastase as estimated in crude neutrophil extracts. These results suggest that neutrophils extravasate and generate vascular leakage without the use of their elastase activity. The inhibitory effect of dextran sulfate on macromolecular leakage may be due to interaction with cationic proteins released from the neutrophils.