Cisplatin, CiS-[PtCl2(NH3)(2)], is known to bind to human serum transferrin, but the binding site remains a matter of some debate. Electrospray ionisation mass spectrometry has been used to characterise the interaction of cisplatin with transferrin. The studies indicate that cisplatin initially docks with, and subsequently bonds covalently to, the hydroxyl functional group of threonine 457, with the loss of HCl affording a transferrin-O-PtCl(NH3)(2) adduct. Copyright (C) 2002 John Wiley Sons, Ltd.