17β-[6,7-3H]Estradiol was incubated with adult human liver slices in Krebs–Ringer phosphate buffer containing glucose. Of the identified 3H recovered, 51–76% consisted of estrone-3-sulfate (E13S) and 17β-estradiol-3-sulfate (E23S). E13S was the main metabolite and was found in both tissue and medium. E23S was present only in the medium. Minor amounts of estrogen glucuronides were formed. When a human liver homogenate was incubated with [3H]E2 in a medium fortified with excess uridine diphosphate glucuronic acid only some 4% of conjugation with glucuronic acid was observed. It is suggested that human liver favors sulfurylation as the conjugating mechanism for E2 and E1.