SK-HEP-1 is an immortal, human cell line derived from the ascitic fluid of a patient with adenocarcinoma of the liver. We have determined that these cells are of endothelial origin. Despite the location of the tumor from which SK HEP-1 was derived, the cell line does not have properties of hepatocytes. Northern blot analysis of total cellular RNA shows no messenger RNA for the hepatic-specific proteins albumin, alpha-fibrinogen, or gamma-fibrinogen. Endothelial characteristics are seen by transmission electron microscopy. These features include numerous pinocytotic vesicles, electron dense granules consistent with Weibel-Palade bodies, and abundant intermediate filaments, identified immunocytochemically as vimentin. Cultures grown on plastic dishes grow in bundles of polygonal to spindle-shaped cells. Proteins characteristic for endothelial cells are identified by immunocytochemistry. Addition of basement membrane material (Matrigel) or type I collagen to the cultures induces these cells to organize into a tubular network.