Dose responses of DNA single- and double-strand breaks (SSB and DSB) of pBR322 irradiated with gamma-rays were measured by gel electrophoresis scanning and the production process of SSB was tested. Results showed that the formation of DSB was related to the free radicals scavenging capacity sigma of mannitol dissolved in DNA solution. The ratio of alphaDSB to betaDSB non-linearly increased with increasing sigma. On the other hand alphaDSB was induced by the single radical transfer mechanism as sigma<10(8)s(-1) it could however be formed by the LMDS mechanism as sigma>2x10(8)s(-1). Moreover the distance for two SSBs forming a DSB in complementary strands was not obviously influenced by the scavenging capacity.