Abstract The sulfation of tyrosine is a common posttranslational modification of peptides and may regulate the biological activity of a peptide. The biological activity of cholecystokinin is dependent on sulfation at the tyrosyl residue. Study of the enzyme(s) responsible for this activation step should provide the basis for future studies of the posttranslational control of peptide biological activity. The experiments reported here were designed to determine the subcellular distribution and biochemical properties of tyrosylsulfotransferase activity in normal human liver tissue. tert-Butoxycarbonylcholecystokinin octapeptide (desulfated) served as the sulfate-acceptor substrate. Subcellular distribution studies showed that tyrosylsulfotransferase activity was localized to the membrane-bound fraction. Tyrosylsulfotransferase was characterized by a pH optimum of 5.8, and activity was increased in the presence of sodium chloride and manganese chloride. The addition of detergents inhibited this enzyme activity. Using the optimized assay conditions, tyrosylsulfotransferase was measured in 20 individual human liver samples. Twentyfold interindividual variation was found—variation that may be related to individual differences in the degree of posttranslational sulfation of proteins. The level of tyrosylsulfotransferase activity did not correlate with the levels of activity of two soluble sulfotransferases, thermostable and thermolabile phenol sulfotransferase. These results provide the basis for future studies to determine the role of sulfation of peptides in humans and whether individual variations in tyrosylsulfotransferase activities correlate with individual differences in the degree of sulfate conjugation of peptides.