H-1-,C-13- and Pt-195-NMR spectroscopies are used to identify the complexes formed between the platinum salts cis-(NH3)(2)PtCl2 (cis-DDP), trans-(NH3)(2)PtCl2 (trans-DDP), cis-(en)Pt(ONO2)(2), and [(dien)PtBr]Br and the tetrapeptide Boc-Cys(1)(SMe)-Ser(2)-Ala(3)-Cys(4)(SMe)-CONH2 (CSAC) containing the sequence Cys-X-Y-Cys (X, Y = amino acids) and being a model of metallothionein (MT) and/or a model for platinum binding to methionine type sulfur, known to occur in biological systems. MT, rich in cysteine is known to bind both in vivo and in vitro with the antitumor drug cis-DDP. The H-1- and C-13-NMR assignments were made by two-dimensional homo-and heteronuclear experiments for the ligand CSAC. The S-CH3 groups coordinate through sulfur to Pt(II) in all cases. The results show that cis-DDP forms a mixture of different diastereoisomers around the sulfur chiral centers and/or polymeric species with NH3 liberation, due to the strong trans-effect of sulfur. cis-Pt(en)(ONO2)(2) forms a monomeric (1:1) chelate structure with CSAC, without en liberation, coordinated through both sulfur atoms. However, slow en liberation could take also place upon increasing temperature. Three signals are observed in the H-1- and Pt-195-NMR spectra of this complex in accordance with the proposed monomeric structure. trans-DDP, on the other hand, forms a 2:1 complex with CSAC identical to the one formed by [Pt(dien)Br]Br, both coordinated to the -S-CH3 groups. No amine release was observed in the case of these two complexes.