Chemical and biological studies are presented for a new series of platinum(II) antitumor agents that violate the classical structure-activity relationships established for platinum complexes. These new agents, which have demonstrated activity against murine and human tumor systems, are cis-[Pt(NH3)2(Am)Cl]+ cations, in which Am is a derivative of pyridine, pyrimidine, purine, or aniline. Members from this series block simian virus 40 DNA replication in vitro and inhibit the action of DNA polymerases at individual guanine residues in replication mapping experiments. Monoclonal antibodies that bind selectively to cisplatin lesions on calf thymus DNA were used in a competitive enzyme-linked immunosorbent assay study to show that the platinum-triamine complexes do not produce the type of intrastrand cross-links on DNA that are characteristics of cisplatin and analogues with the general formula cis-[Pt(amine)2X2]. These results indicate that cis-[Pt(NH3)2(Am)Cl]+ cations form monofunctional adducts on DNA rather than eliminate NH3 or Am to afford bifunctional lesions. This conclusion is further supported by nuclear magnetic resonance spectroscopic and enzymatic digestion analyses of the products of the reactions of these triamine complexes with d(GpG) and dG, which also reveal monofunctional binding. When cis-[Pt(NH3)2(4-Br-pyridine)Cl]+ was allowed to stand in phosphate-buffered saline at 37 degrees C for 14 days, however, NH4+ was released and trans-[Pt(NH3)(4-Br-pyridine)Cl2] formed concomitantly. This compound was characterized by a single crystal X-ray diffraction study, the details of which are reported. The fact that trans-[Pt(NH3)(4-Br-pyridine)Cl2] displays no anticancer activity, however, indicates that its formation from cis-[Pt(NH3)2(4-Br-pyridine)Cl]+ is not a significant component of the mechanism of action of this platinum-triamine complex. Taken together, these findings indicate that the cytotoxicity of cis-[Pt(NH3)2(Am)Cl]+ complexes most likely arises from the formation of monofunctional adducts. The DNA binding properties associated with this new class of antitumor agents suggest that they may display an activity profile different from that of cisplatin and related analogues.