DNA lesions, especially DNA double−strand breaks (dsbs), are looked upon as the dominant molecular effect of radiation action. Dsbs mark the beginning of a cascade of cellular processes that either results in complete repair of the DNA damage or lead to deleterious stages such as mutation, transformation or even cell death. Changing the radiation quality can influence the radiosensitivity of cells in culture. Accelerated particles provide an excellent means of varying the ionization density of the test radiation. With ion beams, the molecular mechanisms underlying the biological consequences of high linear energy transfer (LET) irradiation can be studied and describing radiation action with biophysical models can be tested. In this paper, radiation−induced DNA double−strand breaks (dsbs) were measured in hepatoma SMMC−7721 cells by means of an experimental approach involving pulsed−field gel electrophoresis and densitometric scanning of ethidium bromide stained gels. With this set−up, the induction of dsbs was investigated in SMMC−7721 cells after irradiation with accelerated carbon ions with specific LET 70keV/μm. The fraction of DNA retained was taken as quantitative measure to calculate absolute yields of induced DNA dsbs. Experimental data shows that the induction of DNA dsbs increasing with the dose of irradiation. Data are compared with published results on dsb induction in mammalian cells by radiations of comparable LET.