Decay-accelerating factor (DAF) has been previously described only in cells of bone marrow origin where it serves as a negative modulator of complement activation. Using mAb against human DAF, we demonstrated the presence of DAF in human umbilical vein endothelial cells by immunofluorescence microscopy and flow cytometry. By means of an immunoradiometric assay we detected an average of 3.3 X 10(5) molecules of DAF on each cell. When immunoisolates were analyzed in Western blots, endothelial cell DAF comigrated with DAF purified from normal erythrocytes. DAF was synthesized by the endothelial cells since 35S-labeled DAF could be immunoisolated from HUVEC cultured in medium containing [35S]methionine. This is the first evidence for the presence of DAF in cells of extra-marrow origin. DAF may protect endothelial cells from complement-mediated injury.