It is well known that MCF-7 cells, when incubated with hydroxytamoxifen (OH-Tam) loose their capacity to bind [3H]estradiol. By using Western blotting and [3H]tamoxifen aziridine labeling of KCl extracts from these cells we found that this loss in binding capacity was not associated with a disappearance of the estrogen receptor (ER) protein, an event known to occur after incubation with estradiol. Attempts to label under exchange conditions these ER molecules, which, on the basis of enzyme immunoassays appear to accumulate under OH-Tam treatment, were unsuccessful. Cell fractionation suggested that their origin is nuclear. Assessment of a few triphenylethylenic antiestrogens, as far as their inhibitory potency towards the in vitro MCF-7 cell growth is concerned, indicated a correlation between accumulation of these non-binding ER molecules and the antiestrogen antiproliferative action. However, we were unable to demonstrate absence of such an ER accumulation in two tamoxifen-resistant variants. Impaired folding of the ER protein or impaired phosphorylation of its hormone-binding domain are attractive hypotheses to account for these non-binding ER molecules. Whether these ER molecules have any physiological role, such as competition with the "normal" receptor molecules for the estrogen responsive elements on the DNA is unknown and deserves further study.