Cisplatin, cis-[PtCl2(NH3)2], is commonly utilized in various combination chemotherapy protocols for the treatment of both ovarian and breast cancer while the corresponding trans isomer is therapeutically inactive. This work describes efforts to elucidate the cellular mechanism of action of a novel trans-platinum compound, trans-(dichloroamminethiazole)platinum(II) (ATZ), which demonstrates antiproliferative and cytotoxic effects against both MCF-7 human breast and A2780 human ovarian carcinoma cells in culture. A2780 cells were approximately twofold more sensitive to ATZ than MCF-7 cells in both cell growth and clonogenic survival assays. Dye exclusion studies revealed a 50-70% loss in cell viability within the first 12 h of drug treatment in both cell lines. This initial wave of cell death was succeeded by a prolonged interval of growth arrest during which a small fraction of apoptotic cells was detected. Binding of ATZ to DNA, as estimated by atomic absorption spectroscopy, was similar for the two cell lines and was almost completely reversed 24 h after drug removal. ATZ also induced DNA strand breakage as well as DNA-protein crosslinking during the initial 12 h period when the bulk of cell death was evident. However, neither the extent of DNA strand breakage nor that of DNA protein crosslinking was sufficient to explain the different drug sensitivity in the two cell lines. At 24 and 48 h after exposure of MCF-7 cells to high concentrations of ATZ, the formation of DNA-topoisomerase I complexes is detected, coincident with a high degree of apoptosis. These studies suggest that ATZ has the capacity to interfere with topoisomerase I in the tumor cell, a function not evident in cis-platinum-based drugs.