We investigated the role of the cadherins 5 and 13 in the solute barrier formed by aortic endothelial cells in vitro. In confluent monolayers of bovine aortic endothelial cells, immunofluorescence with antibodies to the external domain of cadherin 5 (Mab 9H7) or to cadherin 13 (Mab Ec6C10) found staining for both cadherins at endothelial cell borders. Western blotting with an antibody to the characteristic cadherin cytoplasmic tail or with an antibody to the extracellular domain of cadherin 5 revealed a single 125 kD protein band. A second larger band was found at 130 kD with the anti-cadherin 13 Mab which was not recognized by an antibody to the cadherin cytoplasmic tail. A calcium switch strategy was used to investigate the involvement of these cadherins in the endothelial barrier. Changes in the permeability of small solutes in an endothelial cell column produced by a decrease in calcium concentration followed by a return to normal calcium, with or without antibody, were recorded. We found that anti-cadherin 5 IgG (10 micrograms/ml) interfered with the reforming of interendothelial junctions after restoration of calcium at every time point tested for a total of 45 min after restoration of calcium. The anti-cadherin 13 IgG (10 micrograms/ml) did not block reforming of the endothelial barrier in a similar manner. The presence of this antibody delayed only by 15 min the restoration of the normal barrier. Without calcium switch, addition of either monoclonal antibody (10 micrograms/ml) to the endothelial cell column had no effect on solute permeability. These results suggest that cadherin 5 in bovine aortic endothelial cells has a major functional role in forming the calcium-sensitive endothelial junction in vitro and may play an important role in the normal structure and function of the in vivo barrier.