Sulfation of human coagulation factor V was investigated by biosynthetically labeling the products of HepG2 cells with [35S]sulfate. There was abundant incorporation of the sulfate label into a product identified as factor V by immunoprecipitation, lability to proteases, affinity for the lectin jacalin, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Two or more sites in factor V incorporated sulfate as indicated by labeling of different peptide chains of factor Va. The 150-Kd activation fragment of factor Va incorporated the greatest amounts of sulfate. This fragment of factor Va was bound selectively by jacalin-agarose, reflecting its content of O-linked oligosaccharides. Analysis of an alkaline hydrolysate of sulfate-labeled factor Va by anion-exchange chromatography showed that the sulfate occurred partly in tyrosine sulfate residues and partly in alkaline-labile linkages. Sulfate groups are potentially important structural and functional elements in factor V, and labeling with [35S]sulfate provides a useful approach for examining the biosynthesis and processing of this protein. The hypothesis is advanced that sites of sulfation in factor V and several other plasma proteins contribute to the affinity and specificity of thrombin for these molecules, just as it does for the interaction of thrombin with the potent inhibitor hirudin from leeches.