The formation of "vessel-like" structures has been previously reported in cultures of endothelial cells from both large and small vessels. In this study the formation of such tubular structures was observed in post-confluent cultures of bovine retinal microvascular endothelial cells. As the cells became postconfluent, cell dropout occurred and the remaining islands of cells were observed to be interconnected by cords. Electron microscopy demonstrated that each cord consisted of a continuous lumen surrounded by one or more endothelial cells. The mechanisms of in vitro "vessel formation" by retinal endothelial cells do not appear to relate to either those of embryonic retinal vascularisation or those associated with retinal neovascularisation and may be an unrelated phenomenon only expressed in vitro. © 1990 Springer-Verlag.