The capacity of human kidney T-1 cells to rejoin DNA breaks induced by accelerated heavy-ion beams of <tex-math>${\rm C}^{6+}$</tex-math>, <tex-math>${\rm Ne}^{10+}$</tex-math>, and <tex-math>${\rm A}^{18+}$</tex-math> (308 to 500 MeV/amu) was studied. Cell monolayers were irradiated on ice with 2000 rad at various positions in the unmodified Bragg ionization curve. The data show that as the LET increases, the rate of rejoining becomes substantially slower than that normally found for X rays. The impaired rejoining capacity becomes maximal in the 100 to 200 keV/μm range where 25% (±6.8%) of the initial number of breaks per cell do not rejoin. In comparison, the induction of a maximal number of unrejoined breaks <tex-math>$\text{cell}^{-1}\ \text{rad}^{-1}$</tex-math> and cell-inactivation studies made under the same experimental conditions show a maximal biological effectiveness at about 100 keV/μm. The data were evaluated both in terms of ionization densities expressed as LET (keV/μm) and in terms of the factor <tex-math>$(Z^{\ast})^{2}/\beta ^{2}$</tex-math>, where Z* is the charge of the stripped nucleus and β is the ratio of its velocity to the velocity of light.

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