We report here the synthesis and cell-proliferation properties of derivatives of the breast cancer drug tamoxifen, in which the -O(CH(2))(2)N(CH(3))(2) side chain, responsible for the drug's antiestrogenic properties, has been modified by a ferrocenyl moiety. We recently reported the diphenol compound 5, in which this amino chain had been replaced with an acyl-ferrocenyl (-O(CH(2))(2)C(O)[(eta(5)-C(5)H(4))FeCp]) group, and which showed antiproliferative effects against both the hormone-dependent MCF-7 and -independent MDA-MB-231 breast cancer cell lines. We now report the results of a structure-activity relationship (SAR) study, in which the lateral chain length has been varied, the ketone group has been omitted, and the number of phenol groups has been varied. Compounds 1-4, with a side chain lacking the carbonyl function (-O(CH(2))(n)[(eta(5)-C(5)H(4))FeCp], n = 1-4) and which show a decreasing affinity for ERalpha (ER = estrogen receptor) with increasing chain length, act as estrogens on MCF-7 cells, and mild cytotoxics on PC-3 prostate cancer cells, with IC(50) values around 10 microM. The two monophenolic derivatives of 2, 2 a and 2 b, which show a reduced affinity for ERalpha compared to 2, are also estrogenic, but are only slightly cytotoxic. Finally, we have reexamined compound 5 and discovered that its antiproliferative effect against the MCF-7 cell line does not arise from antiestrogenicity as we had originally suspected, but by means of a cytotoxic pathway. This compound is also sensitive to the number of phenol groups as cell death is diminished when one of the hydroxyl groups is omitted (5 a and 5 b). Molecular modeling studies of the ligand-ERalpha binding stability are broadly consistent with the experimental binding affinity results for compounds 2, 2 a, 2 b, 5, 5 a, and 5 b. Electrochemical experiments show that 1-4, 2 a, and 2 b are stable to oxidation on the electrochemical timescale, unlike 5, 5 a, and 5 b, and that cytotoxicity is related to less positive phenol oxidation potentials. The SAR study shows that the presence of a ketone group and two phenol groups is necessary for strong receptor binding and cytotoxic effects, and that all compounds are estrogenic, despite the presence of a bulky side chain.