Chang liver cells, Caco-2 human intestinal epithelial cells and Madin-Darby canine kidney (MDCK) cells, labelled with [35S]sulphate in the presence of different concentrations of cycloheximide, produced 87.7-95.3%, 35.8-41.1% and 23.2-25.9%, respectively, of the amounts of free tyrosine O-[35S]-sulphate (Tyr[35S]) formed by corresponding cells labelled in the absence of cycloheximide. Homogenates prepared from the three kinds of cells showed the presence of enzymic activities catalysing the sulphation of L-tyrosine, with specific activities in the order: Caco-2 cells > MDCK cells > Chang liver cells. In all three cases, most of the tyrosine sulphotransferase' activity was found in the cytosolic fraction, indicating the enzyme to be a cysolic protein. A tyrosine-dependence experiment revealed that, for all three kinds of cells labelled with [35S]sulphate, the production of free Tyr[35S] was proportional to the concentration of L-tyrosine present in the culture medium. These results imply an involvement of sulphation in removing excess intracellular L-tyrosine.