Selective estrogen receptor (ER) modulators (SERMs) are important therapeutic agents for breast cancer prevention and treatment. We have synthesized two analogs, E11-2,1 [methyl-(3,17β-dihydroxyestra-1,3,5(10)-triene-11β-yl)acetate] and E11-2,2 [ethyl-(3,17β-dihydroxyestra-1,3,5(10)-triene-11β-yl)acetate], the methyl and ethyl esters of an estradiol analog, substituted in the B ring at C-11β with a carboxymethyl group. The shorter methyl ester, E11-2,1, has high ER affinity and high estrogenic potency in the Ishikawa estrogen cell bioassay, whereas the longer ethyl ester, E11-2,2, has even higher ER affinity, but little or no estrogenic activity. We found that this minor change of one methylene group transforms a potent estrogenic agonist into an antagonist in vitro with either ER α or β. In the rat, E11-2,2 acts as a SERM in the uterus, where it inhibits estradiol-induced proliferation, and as an estrogen agonist in the liver and skeleton, where it decreases plasma cholesterol and increases bone growth. The characteristic feature of antiestrogens, including SERMs, is a long and polar side-chain that prevents agonist-induced conformation of helix 12 of ER. E11-2,2 with its short, nonpolar side-chain, lacks this critical structure, presenting the possibility that it might act through a unique mechanism.