Homogenates of human vascular endothelial cells catalyze the transfer of 35SO4 from 3'-phosphoadenosine 5'-phospho[35S]sulfate into macromolecular endogenous acceptors with properties of both glycoproteins and glycosaminoglycans. Analysis of the 35S-glycoprotein products by both DEAE-cellulose chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicates that these 35S-glycoproteins correspond to the major sulfated glycoproteins observed when endothelial cell cultures were labeled with 35SO4 [A. Heifetz, C. Watson, A. R. Johnson, and M. K. Roberts (1982) J. Biol. Chem. 257, 13581-13586]. The pH optimum of the glycoprotein sulfotransferase is 7.0-7.5, which is distinctly different than that of endothelial glycosaminoglycan sulfotransferase(s), whose pH optimum is 6.0. The 35S-glycoprotein products were characterized as sialated, triantenary-branched, N-linked 35SO4-oligosaccharides containing an endo-beta-N-acetylglucosaminidase D-sensitive site. The site of sulfation was characterized as the GlcNAc residue on the reducing end of the N-linked oligosaccharide. Thus, these sulfotransferases in a cell-free homogenate appear to preferentially add sulfate moieties to a specific class of glycoprotein acceptors at a specific site on sialated oligosaccharides.