Transcobalamin II (TCII) is a cobalamin (Cbl) binding protein in the plasma that mediates the cellular uptake of Cbl. Although the synthesis of TCII by a variety of cultured mammalian cells and by some isolated perfused organs has been reported, no single tissue has been identified as the source of TCII in vivo. In this study, we demonstrate that cultured human umbilical vein endothelial cells secrete a protein that binds CN[57Co]Cbl, elutes from a Sephacryl S-200 column in the same position as TCII, and precipitates with an antiserum to purified human TCII. The biosynthesis of TCII by these cells was confirmed by demonstrating the incorporation of [35S]methionine into a nascent protein that immunoprecipitated with anti-TCII and which, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) had an Mr of 43,000, the same as human TCII. This secreted protein also had the functional properties of TCII because it facilitated the uptake of CN[57Co]Cbl by the same endothelial cells that secreted it as well as other cell lines that express the membrane receptor for TCII. We also present evidence that the venous endothelium could be the source of TCII in vivo by showing that an intact umbilical vein in an isolated umbilical cord, when perfused with medium containing [35S]methionine, secretes a radiolabeled nascent protein with the same immunoreactive and electrophoretic properties as human TCII. These studies demonstrate that the endothelial cell, which has been shown to secrete a number of plasma proteins, also synthesizes and secretes TCII both in vitro and as an intact endothelium in situ, and therefore, could be the source of circulating TCII in vivo.