Capillary endothelial cells cloned from bovine and human tissues were grown in long-term culture. In the presence of tumor-conditioned medium, capillary tubes formed when the cells became confluent. Each tube began as a longitudinal vacuole in one cell and then appeared to be extruded and connected from one cell to the next. Branches appeared, and a capillary network developed over a period of 2 to 3 weeks. By light and electron microscopy, the tubes were nearly identical to capillaries in vivo. These experiments:(1) Demonstrate angiogenesis in vitro;(2) Show that all the information necessary to develop an entire capillary in vitro can be expressed by one cell type;(3) Suggest a mechanism for lumen formation by capillaries in vivo;(4) Offer a possibility of distinguishing between direct and indirect angiogenesis factors.