Extracellular matrix (ECM) components phenotypically modulate cultured endothelium. This paper examined the ability of ECM components to promote tube formation in vitro. When collagen type I was added to the culture medium of confluent neonatal foreskin or human umbilical vein endothelium at 10-100 micrograms/ml tube-like structures formed rapidly. Tube formation did not occur with the addition of other ECM components at similar concentrations. Electron microscopy revealed that the lumen of the tubes consisted of collagen fibrils, with the surrounding cells having typical endothelial junctional complexes. These findings demonstrate that when collagen fibrils make contact with the apical side of endothelium they act as a stimulus and provide a template for vascular tube formation.