Human placenta subfractions were incubated with radioactive oestriol and 16-oxo-oestradiol. 16-oxo-oestradiol was identified from the oestriol incubation. Oestriol and 16-epi-oestriol were characterized from the incubation while 16-epi-oestriol were characterized from the incubation with 16-oxo-oestradiol. The 16alpha-dehydrogenase has been located in the soluble fraction (105 000 g supernatant) of the human placenta.