Cisplatin, raceme-diaqua[1,2-bis(4-fluorophenyl)ethylenediamine]platinu m(II) sulfate (compound I), meso-diaqua[1,2-bis(4-fluorophenyl)ethylenediamine]-platinum(II) sulfate (compound II), and meso-diaqua[1,2-bis(2,6-dichloro-4- hydroxyphenyl)ethylenediamine]platinum(II) sulfate (compound III) were compared with regard to their effect on the MCF-7 breast cancer cell line in vitro. At equimolar concentrations (5 microM), cisplatin, compound I, and compound II were equiactive after 231 h drug exposure, whereas compound III was ineffective. Although compounds I and II showed markedly greater inactivation than did cisplatin after 6 h incubation with culture medium, compound I (but not compound II) exhibited antitumor activity equivalent to that of cisplatin when cells were exposed to the drugs for 6 h. Platinum measurements by neutron-activation analysis revealed that compound I was selectively and rapidly accumulated by MCF-7 cells, resulting in a high degree of DNA platination within the first few hours of drug exposure. However, when the drug-exposure period was long enough, platinum enrichment was not reflected in an overall difference in the cytotoxicity of compound I vs cisplatin. Nevertheless, compound I should be superior to cisplatin in vivo, provided that effective plasma levels can be maintained for about 6 h.