1. Occurrence of lesions induced in plasmid DNA by cis-DDP and by HA was quantified both as a transforming activity and as conformation integrity of supercoiled pBR322 DNA. Fifty per cent decrease of the biological activity of plasmid DNA, not accompanied by measurable change of DNA conformation, was observed after a single exposure of DNA to cis-DDP (1 hr/37 degrees C). 2. HA induced conversion of supercoiled DNA to other topological forms in a dose-dependent manner. 3. One- and two-strand DNA breaks were determined electrophoretically with high sensitivity. Cis-DDP exposed DNA relaxed at 30 times lower HA concentration compared to intact DNA. 4. This effect may be connected with a local distortion of DNA structure at the cis-DDP--DNA bond, which makes possible high effectivity of HA-DNA interaction. 5. On other hand, biological activity stayed at the 50% level despite breaks induced in DNA. 6. This finding supports the idea that DNA breaks occur at the locations which were modified during the exposure of DNA to cis-DDP. 7. The importance of the DNA structure during interaction with HA may be seen during HA-DNA interaction at heat-denaturation of supercoiled DNA. At this condition, the DNA breaks were induced at 100 times lower concentration of HA. 8. We conclude, on the basis of these results published earlier, that local distortion of supercoiled DNA structure, which is caused by the cis-DDP bond, and the local DNA uncoiling caused by heat-denaturation are related to high HA-DNA reactivity.