Interactions of metal-based drugs with proteins and consequent adduct formation (the so-called "protein metalation" process) play a key role in the mode of action of several anticancer agents and in determining their toxicological profile. Through a novel investigative strategy grounded on the combined use of electrospray ionization mass spectrometry (ESI MS) and biological macromolecule X-ray crystallography we show that it is possible to clarify in depth the metalation process of small model proteins; a number of instructive examples are provided. Recently, this kind of investigative approach has been extended to bigger proteins such as human serum albumin and horse spleen ferritin, with rather encouraging results. Overall, by application of this strategy, metalation of proteins caused by anticancer metallodrugs can be disclosed in the molecular detail.