The glucuronyltransferase activity towards morphine was measured in microsomes isolated from liver specimens obtained from human fetuses and cancer patients. All the fetal livers investigated had measurable UDP-glucuronyltransferase activity towards morphine. There was no correlation between the gestational age (15 to 27 weeks) and the glucuronidation rate. The mean value of the enzymatic activities was higher in fetal livers obtained by hysterotomy (0.20 nmoles x min-1 x mg-1) than in livers obtained after induced abortion (0.11 nmoles x min-1 x mg-1). The average rate of glucuronidation in microsomes from adult liver (mean 1.15 nmoles x min-1 x mg-1) was 6 to 10 times higher than in the fetal liver microsomes. Together with previous investigations on human adult and fetal liver glucuronidation, the present results support the theory of heterogeneity of human UDP-glucuronyltransferase.