This study describes the ability of bovine corneal endothelial cells to proliferate and organize monolayers when cultured in the presence of a saline-soluble fraction of the retina (RE). This strongly stimulates cell proliferation by increasing the growth rate. In the continuous presence of RE, the cells organize a confluent monolayer composed of closely apposed polygonal cells lying on a basement membrane. Thus, over a large number of generations, these cells express most of their differentiated traits: contact inhibition of movement, proliferation arrest at monolayer formation, and the extracellular deposition of a material organized as a typical basement membrane. The presence of RE increases by a factor of 10 the number of cellular generations expressing these differentiated traits, and subcultured cells maintained in a medium lacking Re are unable to build up a similar organization at confluency, even if this is extended over several months. If these cultures are transferred to a medium containing RE, they behave like endothelial cells which have been always cultured with RE. We suggest that the expression of the endothelial differentiated state is related to the raised mitotic activity induced by the retinal factor.