Sulphate conjugation of biogenic amines and phenolic drugs is mediated by the enzyme phenolsulphotransferase (PST). A detailed examination of the properties of PST in several different human tissues using a variety of substrates has been carried out in an attempt to answer some basic questions about the human enzyme: 1 ) is the same enzyme responsible for conjugating endogenous neurotransmitters as for exogenous phenolic drugs, 2 ) do the properties of the human enzyme closely resemble those previously studied in rat tissues, 3) are the properties of the enzyme similar in different human tissues, 4) can the platelet enzyme be used as a highly accessible model tissue for clinical studies of sulphate conjugation in the brain and throughout the rest of the body and 5) can cells derived from human neural crest tumours when grown in tissue culture be used as a model tissue for the human enzyme.