Various oestrogen responsive reporter genes and vectors expressing truncated or chimeric human oestrogen receptors (hER) containing either of the two independent hER transcriptional activation functions (TAF‐1 and TAF‐2) have been transfected into HeLa cells, chicken embryo fibroblast (CEF) or yeast cells to investigate the agonistic activity of the anti‐oestrogen 4‐hydroxytamoxifen (OHT). We demonstrate that the agonistic effect of OHT on the whole hER is due to the cell‐type and promoter‐context dependent activity of TAF‐1. In similar experiments, we show that the anti‐oestrogen, ICI 164,384, does not exhibit any oestrogenic activity and, therefore, acts always as a pure antagonist, even though it does not inhibit the activity of the isolated TAF‐1. We also confirm that the wild type human oestrogen receptor has no ligand independent transcriptional activity. The implications of our results for the variable antagonist/agonist activity of anti‐oestrogens in vivo are discussed.