In this paper we explore cisplatin interactions with sulfur-containing amino acids in a polarizable continuum model. Two cisplatin hydrated complexes were considered as reactants (chloro complex, cis[Pt(NH3)(2)Cl(H2O)](+); hydroxo complex, cis-[Pt(NH3)(2)(OH)(H2O)](+)). We considered the following reaction mechanism: first step, substitution of the aqua ligand by amino acid; second step, dissociative chelate formation. For the optimized complex (at the B3LYP/6-31+G(d)/COSMO level), the energy profile was determined using the B3LYP/6-311++G(2df,2pd) level and two different PCM models-COSMO and UAKS/DPCM methods which were adapted for use on transition metal complexes. The results show thermodynamic preference for bonding by cysteine sulfur followed by the amino group nitrogen, methionine thioether sulfur, and carboxyl-group oxygen. Methionine slightly prefers the Pt-N(Met) coordination in the chloro complex, but in the hydroxo complex it prefers the Pt-S(Met) coordination. A similar trend follows from the bonding energies: BE(Pt-S(Cys)) = 80.8 kcal/mol and BE(Pt-N(Met)) = 76 kcal/mol. According to the experimental observations, the most stable structures found are kappa(2)(S,N) chelates. In the case of methionine, the same thermodynamic stability is predicted also for the kappa(2)(N,O) chelate. This differs from the gas-phase results, where kappa(2)(S,N) and even kappa(2)(S,O) were found to be more stable than kappa(2)(N,O) complex.