The interactions with protein targets of the ruthenium(III) complex imidazolium trans-[tetrachloro(dimethyl sulfoxide)(imidazole)ruthenate(III)], NAMI-A, an effective anticancer and antimetastatic agent now in clinical trials, deserve great attention as they are believed to be at the basis of the mechanism of action of this innovative molecule. Here, we report on the reactions of NAMI-A with two well-known model proteins, namely, hen egg white lysozyme and horse heart cytochrome c; these reactions were investigated by a variety of physicochemical methods, including optical spectroscopy, 1H NMR and electrospray ionization mass spectrometry. The combined use of the analytical techniques mentioned resulted in a rather exhaustive description of the NAMI-A–protein interactions; in particular, the formation of fairly stable metal–protein adducts was clearly documented and the nature of the resulting protein-bound metallic fragments ascertained in most cases. Notably, greatly different patterns of interaction were found to be operative for NAMI-A toward these two proteins. The biological implications of the present findings are discussed.