In an effort to prepare effective nonsteroidal antiestrogens without intrinsic estrogenicity and with greater antagonism than the triarylethylenes (tamoxifen), four (E)-and (Z)-1,1dichloro-2-phenyl-2-[4-(2-diethylaminoethoxy)phenyl]-3-(4-methoxyphenyl)cyclopropane analogs of the antiestrogen MER 25, of which two of the compounds had a 4-heptafluorotolyl group in the Ν°-ring, were prepared. The (E)-and (Z)-gem-dichlorotriarylcyclopropanes were tested for their ability to inhibit the growth of estrogen receptor (ER)-positive MCF-7E3 and ER-negative MDA-MB-231 human breast cancer cells in culture. All compounds, except 18E, exhibited a statistically significant (P Ο½ 0.01) reduction in estradiol-stimulated growth (antiestrogenic activity) at 1.0 ΘM concentration in the MCF-7E3 cells: 11Z (88%), 11E (106%), 18Z (65%), and the test compounds 7A(Z) (85%), 7A(E) (53%), MRL 37 (91%), MER 25 (71%), and ICI 182,780 (102%). Inhibition of estradiol-stimulated growth at concentrations lower than 1.0 ΘM was demonstrated by 11E, MER 25, and ICI 182,780. Compound 11E produced weak inhibition at 0.1 nM (19%) and nearly complete inhibition (79-112%) over a concentration range of 1.0 to 100 nM. MER 25 produced inhibition of estradiol-stimulated growth at 1.0 (39%), 10 (102%), and 100 nM (100%) concentrations. ICI 182,780 completely inhibited estrogen-stimulated growth from 0.1 nM to 1.0 ΘM concentrations. Two compounds exhibited estrogenic activity: 18E (from 1.0 nM to 1.0 ΘM concentrations) and MER 25, which had antiestrogenic action at the lower concentration ranges, but exhibited estrogenic properties at 100 nM to 1.0 ΘM concentrations. None of the test compounds or standards were active in the MDA-MB-231 cell line at the concentrations studied (0.01 nM to 1.0 ΘM). In addition, none of the compounds inhibited cell growth below control in the MCF-7E3 cell line. The results from both cell lines suggest that the test compounds are devoid of any antitumor properties, which is thought to be mediated through a nonreceptor mechanism. Analog 11E has the potential to be useful in the treatment of hormone-responsive breast cancer.