The effect of the novel and potent monofunctional platinum(II) agent phenanthriplatin on Escherichia coli and bacteriophage lambda lysogens is reported. E. coli filamentation was observed by light microscopy when cells were grown in the presence of phenanthriplatin, cis-[Pt(NH3)(2)(Am)Cl](+) where Am is phenanthridine. Treatment of lysogenic bacteria with this compound resulted in lysis and the production of viral particles, as indicated by plaque formation in a bacterial lawn. The results obtained with phenanthriplatin are contextualized by comparison with those obtained using cisplatin as well as other, less active, monofunctional compounds such as [Pt(NH3)(3)Cl](+) and cis-[Pt(NH3)(2)(py)Cl](+), where py is pyridine. The ability of phenanthriplatin to induce bacterial filamentation and initiate lysis in lysogenic bacteria corroborates the hypothesis that the biological activity of this complex is mediated by its interaction with DNA.