Résumé : Linkage of a 11β-chloromethyl group to estradiol-17β (E2) dramatically increases the binding affinity of the steroid for the estrogen receptor (ER) with the formation of a quasi-irreversible steroid-receptor complex. We have synthesized the two isomers of 11β-chloromethyl-17α-iodovinyl-estradiol (E-CMIV and Z-CMIV) by a novel route. Both derivatives demonstrated high binding affinity and selectivity for ER (RBAs: ER = 820 and 1008; SHBG = 1.2 and 0.25, respectively; E2 = 100). On the basis of X-ray crystallographic data for Z-CMIV and its precursor, we have postulated that Z-CMIV might interact strongly with aromatic amino-acids within a hydrophobic groove of the ER hormone binding domain (HBD) that incorporates pockets corresponding to the 11β and 17α steroid substituents. The binding properties of Z-CMIV labeled with 125I were investigated, especially its ability to detect and quantify altered ER forms with low binding affinity for E2. Sucrose density gradient analysis revealed that Z-CMIV has a higher activation potency than E2 as it converts a higher proportion of non-activated monomers in the cytosol into activated monomers with the potential to dimerize. In tit vitro (MCF-7 cells) and in vivo (rat uterus) determinations of estrogenic activity, Z-CMIV was as potent as E,in increasing progesterone receptor (PgR) concentrations and decreasing ER levels and in stimulating uterine growth. [125I]-Z-CMIV could open the way to new applications in the diagnosis and therapy of ER-positive breast cancers, especially those containing altered (variant) ERs.