Human blood vessels and human umbilical vein endothelial cells in culture were examined for the presence and synthesis of factor V. Factor V antigen was detected on the luminal surface of blood vessels washed with buffers containing calcium but was absent from segments of the same vessels perfused with buffers containing EDTA. Very low levels of endogenous factor V antigen were found in endothelial cells in culture but these cells did not synthesise factor V in sufficient quantities to be detected by the present methods. Factor V activity was not detected in any of the present cell preparations.