The binding affinity and relative estrogenic potency of 2-bromo-, 4-bromo-, 2-methyl- and 4-methylestradiol was evaluated in MCF-7 breast cancer cells. The relative binding affinities compared to estradiol were 47% for 2-methyl-, 25% for 4-methyl-, 37% for 4-bromo- and 17% for 2-bromoestradiol. However, both 2- and 4-methyl- as well as 2- and 4-bromoestradiol were able (a) to translocate the cytosolic estrogen receptor into the nucleus and (b) to induce the progesterone receptor in a concentration dependent manner. Finally, all ring-A substituted estrogens used in this study induced the pS2 mRNA as demonstrated by Northern-blotting. From these findings we conclude that 2-bromo-, 4-bromo-, 2-methyl- and 4-methylestradiol are agonistic ligands for the estrogen receptor in MCF-7 breast cancer cells.