Phenosulphotransferase (EC 2.8.2.1) (PST) is an important catecholamine and drug metabolizing enzyme. Optimal conditions have been determined for the accurate measurement of PST activity in the human platelet, human renal cortex, and human jejunum with a radiochemical microassay. 3-Methoxy-4-hydroxyphenylglycol (MHPG) and 35S-3'-phosphoadenosine-5'-phosphosulfate (35S-PAPS) were the substrates for the reaction. The apparent Michaelis-Menten (Km) values for MHPG with platelet, renal cortex, and jejunum were 1.09, 0.46 and 1.16 mmol/l, respectively. Apparent Km values for PAPS in the same tissues were 0.14, 0.13 and 0.21 mumol/l. The pH optimum of the reaction in all three tissues was approximately 6.2--6.8 with three different buffer systems. The coefficients of variation for the assay of platelet, renal cortex, and jejunal activities were 6.2%, 3.4% and 4.4%, respectively. Mean platelet PST activity in blood samples from 75 randomly selected adult subjects was 5.0 +/- 1.72 nmol of MHPG sulfate formed per hour per mg of platelet protein (8.3 x 10(-5) +/- 2.9 x 10(-5) mumol . min-1 . mg-1, mean +/- S.D.). There was a 5-fold intersubject variation in platelet PST activity within two standard deviations of the mean value. Experiments in which partially purified human erythrocyte PST was added to platelet, kidney and gut homogenates under these assay conditions provided evidence that endogenous PST inhibitors did not affect the observed enzyme activity.