Two new platinum complexes, trans-chlorodiammine[N-(2-aminoethyl)-4-[4-(N-methylimidazole-2-carboxam ido)-N-methylpyrrole-2-carboxamido]-N-methylpyrrole-2-carboxamide]platin um(II) chloride (DJ1953-2) and trans-chlorodiammine-[N-(6-aminohexyl)-4-[4-(N-methylimidazole-2-carboxamido)-N-methylpyrrole-2-carboxamido]-N-methylpyrrole-2-carboxamid e] platinum(II) chloride (DJ1953-6) have been synthesized as proof-of-concept molecules in the design of agents that can specifically target genes in DNA. Coordinate covalent binding to DNA was demonstrated with electrospray ionization mass spectrometry. Using circular dichroism, these complexes were found to show greater DNA binding affinity to the target sequence: d(CATTGTCAGAC)(2), than toward either d(GTCTGTCAATG)(2), which contains different flanking sequences, or d(CATTGAGAGAC)(2), which contains a double base pair mismatch sequence. DJ1953-2 unwinds the DNA helix by around 13 degrees, but neither metal complex significantly affects the DNA melting temperature. Unlike simple DNA minor groove binders, DJ1953-2 is able to inhibit, in vitro, RNA synthesis. The cytotoxicity of both metal complexes in the L1210 murine leukaemia cell line was also determined, with DJ1953-6 ( 34 AM) more active than DJ1953-2 (> 50 mu M). These results demonstrate the potential of polyamide platinum complexes and provide the structural basis for designer agents that are able to recognize biologically relevant sequences and prevent DNA transcription and replication.