The relationship between deoxyribonucleic acid (DNA) damage and the cell death induced by ¹²C ions irradiation was examined in four kinds of cells, Melanoma B16, cervical squamous carcinoma HeLa, Chinese hamster V79 and hepatoma SMMC-7721. Cell survival was determined by a colonogenic assay, and the sensitivity was described by D₅₀ (the dose of radiation necessary to reduce the survival to 50%). For all cell lines, D₅₀ ranged from 0.74 Gy to 3.85 Gy, among them B16 was the most radiosensitive to ¹²C ions, and V79 and HeLa cells had almost the same radio-sensitivity, SMMC-7721 was the last. The induction of deproteinized DNA double-strand breaks induced by ¹²C ions were measured by pulsed-field gel electrophoresis (PFGE), and the initial yield of the deproteinized DNA dsbs per unit dose was expressed as the DNA double break level (L). A linear dose-response curve was seen for initial DNA dsbs for all cell lines (slopes range from 0.40-0.98 (DSBs/100Mbp/Gy)). V79 was the steepest, B16 was the last. There was an inverse relationship between the initial DNA dsb and D₅₀ if the B16 cell line was not considered, but there was no relativity even excludes the B16 cell line. The present results indicate that there is no relationship between cellular sensitivity and initial DNA dsb, even exclude the effects of chromatin structure.