Résumé : Estrogen receptor (ER), antiestrogen binding sites (AEBS) and caloduln (CaM) are potential targets of antiestrogen (AE) action. To analyse further which of these targets are primarily involved in the antiproliferative activity of these drugs against human breast cancers, two cell clones, namely the RTx6 and LY-2 variants, selected from MCF-7 cells for their resistance to high doses of tamoxifen (TAM) and the Keoxifen (KEG) analog LY 117018, respectively, were studied for their sensitivity to hydroxytamoxifen (OH-TAM) and KEO as well as the strong calmodulin antagonist calmidazolium. The effects of these drugs on both cell growth and progesterone receptor (PgR) concentration were assessed. Binding properties for ER, AEBS and CaM of each compound were also measured. Our results confirmed that basal growth of RTx6 and LY-2 cells was more resistant to OH-TAM and KEO than parent MCF-7 cells, although both displayed a significant inhibition at the highest doses assessed. In regard to calmidazolium inhibition, each variant behaved as did the MCF-7 line indicating that a modification at the CaM level was not responsible for their lower sensitivity to AEs. Nor could the association of CaM to ER which did not differ among all cell lines. Resistance of these variants was not related to AEBS in view of the total lack of such sites in RTx6 cells. However, under estrogenic growth stimulation such sites may play some role, since LY-2 cells in the presence of estradiol displayed a real antiestrogen-resistant pattern while RTx6 cells were more sensitive than MCF-7 cells to OH-TAM. This property was not found in the antagonism against estradiol-induced PgR synthesis which was observed with each variant. Thus the PgR concentration of RTx6 cells was strongly down-regulated by OH-TAM and KEO and reduced in LY-2 cells to the same extent as in MCF-7 cells. All these observations show that AE resistance is not entirely related to ER mediated events and that alterations at the ER and CaM levels are unlikely to account for the lower AE sensitivity of the variants investigated.