A rat brain fraction enriched with microvessels was used as the immunogen to produce mouse hybridoma cell lines secreting monoclonal antibodies. One of these antibodies, selected from 156 supernatants by enzyme-linked immunosorbent and immunofluorescent assays, reacted only with the endothelium of microvessels in the brain. The endothelium-specific antibody labelled the cytoplasm of microvascular endothelial cells, their luminal membranes, and an extracellular layer, the endocapillary coat, which covered the luminal surface of these cells. In the kidney, the antibody specifically stained the brush border of the proximal tubuli, and in the liver, the antibody specifically stained bile canaliculi. This demonstrates the 3 morphological structures with important transport functions, cerebral microvascular endothelium, brush border of kidney proximal tubuli, and liver bile canaliculi, express the same epitope.