In order to determine whether plasmin affects endothelial cell integrity directly, confluent bovine aortic endothelial cells were treated with plasminogen and a plasminogen activator. The permeability of the monolayer to [125I]-albumin was shown to be increased significantly (P < 0.01) with a concomitant decrease in viability. Plasmin activity correlated significantly with endothelial cell permeability (p < 0.004; r = 0.82). Coincubation with D-phenylalanyl-L-prolyl-L-arginyl chloromethylketone, a tripeptide inhibitor of plasmin, reduced the increase in endothelial permeability induced by plasmin by 59% (p = 0.033). Monolayers studied in parallel were stained with rhodamine-phalloidin to visualize F-actin. There were significant morphologic changes in the endothelial monolayers exposed to plasmin compared to control monolayers, and these changes could be attenuated by coincubation with D-phenylalanyl-L-prolyl-L-arginyl chloromethylketone. These studies show that: 1) plasmin induces significant increases in endothelial cell permeability with accompanying morphologic changes; and 2) these deleterious functional and morphologic effects can be attenuated by coincubation with the plasmin inhibitor, D-phenylalanyl-L-prolyl-L-arginyl chloromethylketone.