U-11100A is oestrogenic in vaginal smear tests in rats and mice with a subcutaneous med of ca. 500 μg and 50 to 100 μg respectively. It is also oestrogenic orally and intravaginally and in the tetrazolium reduction test. In all tests, however, the dose-response line turns down again with higher dosage, and full responses were not obtained at the highest dose levels tested. The compound is weakly anti-oestrogenic in both vaginal smear and tetrazolium tests in mice, but exhibits complex interactions with oestradiol, the oestrogen used.