It has recently become clear that a long DNA with a size of more than several tens of kilo base-pairs (kbp) exhibits a large on/off change in conformation between elongated coil and folded compact states. To explore the possible effect of anticancer drugs on the conformation of large DNA, we observed single DNA molecules with regard to changes in their higher-order structure upon the addition of cisplatin and its derivatives, diamminedinitratoplatinum(II) and tetrammineplatinum(II). We found that these cisplatin analogues significantly affected the conformation of individual DNA molecules by inducing a folding transition. (C) 2009 Elsevier B.V. All rights reserved.