Phenol sulphotransferase (PST) activity in rat platelet cytosol for p-nitrophenol (PNP) sulphation was found to have a similar dependence on PNP concentration and thermostability to that in rat liver cytosol. The activities of PST isoenzyme for the sulphation in the microM range of PNP in rat platelets and rat liver were significantly correlated. Thus, measurement of PST activity in platelets could be a useful and practical method for predicting this activity in liver.