In vivo seeding of vascular grafts in dogs has resulted in the formation of a confluent luminal monolayer and increased patency. Results of similar trials in humans have been mixed. Interspecies variation of the harvest, attachment, and growth of vascular endothelium might explain these apparent differences. Endothelial cells were harvested by standard enzymatic techniques from bovine aorta (n = 7), canine external jugular vein (n = 9), and human saphenous vein (n = 8). Growth curves were generated by means of a replicate microwell technique and then compared with the following observations: Harvest of endothelium from human saphenous vein provides a lower yield of viable endothelial cells. When compared to bovine or canine cells, the lag phase of the growth curve of human cells is prolonged and the doubling time in the growth phase is increased. These results suggest that in humans there are fewer available cells, and conditions for endothelial cell growth may be more stringent than for other species. It may be necessary to develop alternative methods of seeding before success in humans can be achieved.