The action of fibrinolytic enzymes (plasmin, miniplasmin, neutrophil leukocyte elastase) on the blood-brain barrier is investigated. The binding and the effects of the fibrinolytic enzymes are studied in the first subcultivation of human brain capillary endothelial cells.125I-labeled plasmin, miniplasmin and neutrophil leukocyte elastase bind to confluent monolayers of cultured endothelial cells with dissociation constants of 1 × 10-8mol/l, 4.8 × 10-7mol/l and 1.8 × 10-8mol/l, respectively, and the number of binding sites varies between 2.3 × 105and 7.5 × 106per cell. Following treatment of the cultured cells with purified and active-site titrated proteases, the changes in morphology of individual cells are analyzed with computerized morphometry. At low concentrations (in nanomolar range) all studied fibrinolytic proteases induce reduction of the cell area; the minimal size is achieved in 20–80 min after the application of an enzyme and the effect is completely reversed in 15 min after its removal. A possible in-vivo consequence of these in-vitro findings is studied in an organ-perfusion model: rat hemisphere is perfused with a protease solution followed by a circulating phase-borne tracer (horse-radish peroxidase). In perfused rat hemisphere, the fibrinolytic enzymes open the blood-brain barrier to the circulation-borne tracer. These results support the concept that fibrinolytic enzymes interact with the brain microvascular endothelium and thus affect the integrity of the blood-brain barrier through active cell contraction.