A rapid assay system for measuring the potential of estrogenic drugs is introduced. Luciferase induction could be measured in estrogen-receptor-positive human MCF-7 breast cancer cells, which had been transfected with a novel luciferase reporter plasmid ERE luc. The minimal requirement was 1 h exposure to the inducing drug and 3.5 h of incubation after removal of the drug. The assay system was used to measure the stability of the drug diaqua-[1,2-bis (2,6-dichloro-4-hydroxyphenyl) ethylenediamine] platinum(II) sulfate, containing an estrogenic ligand and reactive platinum. Luciferase activity was observed only when the drug was in the culture medium and cells for short times, whereas the estrogenic ligand alone remained active. It is assumed that binding of the platinum moiety to macromolecular constituents of the culture or cells renders the drug inaccessible for binding to the estrogen receptor.