Inhalation of diisocyanate vapors is associated with immediate-type hypersensitivity reac tions and direct toxic responses. The genotoxic effects of diisocyanates have not been clarified. The aim of this study was to examine the changes in DNA following in vitro exposure to three most commonly used diisocyanates (toluene diisocyanate, TDI; methyl-enediphenyl-4,4′-diisocyanate, MDI; and hexamethylene diisocyanate, HDI) and to com pare their binding potential using melting behavior of DNA and electrophoresis studies in DNA. Following incubation of DNA with MDI (pure and mix) and HDI we found no dif ferences in the melting behavior compared to the control calf thymus DNA. However, DNA treated with TDI showed differences in the shape of the native DNA curves due to changes in hyperchromicity and exhibited 14% more DNA reconstitution after renaturation. The small changes in the melting behavior of native DNA do not suggest the formation of DNA intrastrand cross-links but rather conformational changes of single- and double-stranded DNA. These conformational changes were further explored by agarose elec trophoresis of native and denatured calf thymus DNA. Control and all diisocyanate-exposed DNA showed no differences in the size of native DNA fragments. Conversely, electrophore sis of TDI mix-incubated DNA, following denaturation, showed a distinct reduction in the double-stranded DNA fragment size compared to the control, MDI-denatured (pure and mix), and HDI-denatured DNA. These findings may help to better understand the mecha nisms of the genotoxic effect of TDI.