Human vascular endothelial cells were cultured from term umbilical cord veins using Medium 199 supplemented with 20% fetal calf serum. Cells initially formed small clusters, spread on plastic or glass, and coalesced into confluent monolayers by day 7. Electron microscopy confirmed their identity as endothelium through the presence of Weibel-Palade bodies. A morphologically distinct subpopulation, identified as vascular smooth muscle cells, was also found in some cultures. Autoradiography showed progressive increases in DNA synthesis in growing cultures, with labeling indices varying between 2.4% and 53.2% depending on cell density. After confluence, labeling became uniform, and localized increases in DNA synthesis occurred in denuded areas, leading to reconstitution of the monolayer. DNA synthesis correlated with cell density in replicate microwell cultures. These findings suggest that endothelial cell cultures can serve as a useful in vitro model for studying endothelial regeneration and pathophysiology. Endothelial cells in culture behave as density-dependent populations with respect to DNA synthesis and can be used to study the mechanisms of endothelial growth and regeneration. The study also identified a second cell type in umbilical vein cultures, which was found to be smooth muscle cells. The isolation and characterization of these cells were critical for understanding endothelial regeneration. The study demonstrated that endothelial cell cultures can be used to study the effects of various factors on endothelial growth and regeneration. The results indicate that endothelial cell cultures provide a useful in vitro model for studying the pathophysiology of endothelial regeneration.Human vascular endothelial cells were cultured from term umbilical cord veins using Medium 199 supplemented with 20% fetal calf serum. Cells initially formed small clusters, spread on plastic or glass, and coalesced into confluent monolayers by day 7. Electron microscopy confirmed their identity as endothelium through the presence of Weibel-Palade bodies. A morphologically distinct subpopulation, identified as vascular smooth muscle cells, was also found in some cultures. Autoradiography showed progressive increases in DNA synthesis in growing cultures, with labeling indices varying between 2.4% and 53.2% depending on cell density. After confluence, labeling became uniform, and localized increases in DNA synthesis occurred in denuded areas, leading to reconstitution of the monolayer. DNA synthesis correlated with cell density in replicate microwell cultures. These findings suggest that endothelial cell cultures can serve as a useful in vitro model for studying endothelial regeneration and pathophysiology. Endothelial cells in culture behave as density-dependent populations with respect to DNA synthesis and can be used to study the mechanisms of endothelial growth and regeneration. The study also identified a second cell type in umbilical vein cultures, which was found to be smooth muscle cells. The isolation and characterization of these cells were critical for understanding endothelial regeneration. The study demonstrated that endothelial cell cultures can be used to study the effects of various factors on endothelial growth and regeneration. The results indicate that endothelial cell cultures provide a useful in vitro model for studying the pathophysiology of endothelial regeneration.