The tracer experiments of Reese and Karnovsky (1) demonstrated that it was the endothelium that formed a permeability barrier because electron-dense tracers such as horseradish peroxidase did not pass from the vessel lumen through the interendothelial cleft. The structure responsible for the lack of permeability of tracers was the tight junction. In endothelial cells, these specialized contact zones were already known from ultrastructural studies (2), and in epithelial cells, their morphology was described in detail by Farquhar and Palade (3).