Different inactivation rates of phage φX 174 were observed between two irradiation methods for charged particles. In method EA, phage particles were spread on a thin film and exposed to air during irradiation. In method CF, the phage sample was covered with a thin polycarbonate film to eliminate the effect of secondary electrons ejected from air in front of the sample. The apparent inactivation efficiency of primary particles determined by method EA was found to be higher than that found by method CF in the case of both α-particles and nitrogen ions. The observed increase in inactivation can be explained by the dispersed energy loss of secondary electrons produced in air as compared with those produced in polycarbonate.

Escherichia coli K-12 AB1157 ( ${\rm rec}^{+}{\rm uvr}^{+}$ ), AB2500 (uvrA), AB2470 (recB), and JC1553 (recA), which have different DNA repair capacity, were bombarded with α particles, C ions, and N ions having various beam energies in order to determine the LET-dependent radiosensitivity. Radiosensitivity of the uvrA mutant was almost the same as that of the wild-type throughout the whole LET region studied. Difference in radiosensitivity between the ${\rm rec}^{-}$ mutants and the wild-type was found to decrease with increasing LET of charged particles used. Such LET dependence can be explained by the increase in the ratio of irreparable to reparable-type damages in DNA with the increasing LET of the particles. It is also possible that a significant component of the LET-dependent changes in viability observed may be due to damages to the repair system itself.

Actions of alkali halides such as fluoride, chloride, bromide, and iodide were found to enhance the lethal effect of ionizing radiations on cells of bacteria and yeast suspended in 0.067 M phosphate buffer. The data obtained are concerned mainly with the effects of halides present during irradiation. The effects of halide concentration, dose, pH, and freezing on the enhancing actions of halides and also on the aftereffects of irradiation were determined. It was concluded that the possible mechanism for the enhancement by halides may involve the inactivation of a multihit type of short-lived species, which is produced from halide ions by ionizing radiations, but not by ultraviolet light. Different halides, however, appeared to behave differently toward irradiation with respect to forming the longlived species which could have an influence on the enhancement of radiolethality.