Long-term mass culture of human large vessel endothelial cells (EC) from aorta, umbilical cord vein, and pulmonary artery was undertaken and the cells characterized. The cells possessed all the major features of EC, proliferated rapidly [population doubling (PD) time 17-42 hours] and survived 40-60 PD. The cells were cultured under different conditions, and the following combination chosen as optimal for routine culturing: Medium 199, supplemented with 20% human serum, 4 mM glutamine, 200 micrograms ml-1 EC growth factor from human brain, 100 micrograms ml-1 heparin. The cells were grown on gelatin-coated or fibronectin-coated substrate. The results of the experiments with large vessel EC cultivation were used to develop a system for the culture of capillary EC from human lung and kidney. Mass-scale culture of large vessel and capillary EC is valuable in the investigation of EC function and for clinical purposes.