The sulphotransferase and the glucuronyltransferase activities were measured with [3H]ethinyloestradiol in vitro using subcellular fractions from human liver. A rapid and sensitive assay based on a single extraction in ether of the unmetabolized EE2 is described. The mean (+/- SD) of the sulphotransferase and glucuronyltransferase activities from 8 livers was 68.2 +/- 16.5 and +/- 190.4 +/- 82.3 pmol/min/mg, respectively. The kinetic parameters of both enzymes were measured in 4 livers by varying the concentrations of EE2 or the endogenous substrate. At varying concentrations of EE2 the average (+/- SD) of Vmax was 66.5 +/- 15.8 pmol/min/mg (sulphotransferase) and 404 +/- 317 pmol/min/mg (glucuronyltransferase). The mean (+/- SD) of Km was 0.0072 +/- 0.0019 mM (sulphotransferase) and 0.20 +/- 0.12 mM (glucuronyltransferase).