The vegetative cells and spores of the cellular slime mold Dictyostelium discoideum NC-4 are very resistant to 60 Co gamma rays, with a 10% survival dose ( D 10 ) of 300 krad in air. Using ultraviolet light or nitrosoguanidine as a mutagenic agent and replica plating with gamma irradiation as a selective agent, we have isolated several stable daughter strains that are more sensitive to gamma rays. A comparison of some of the radiation responses of the parent (NC-4; wild type), γs-18 ( ${\rm D}_{10}=75\ {\rm krad}$ ), and γs-13 ( ${\rm D}_{10}=4\ {\rm krad}$ ) strains has been made. All three strains show the same gamma-ray-induced division delay of about 1 cell generation time for 54 krad. Strains wt (wild type) and γs-18 show split-dose recovery of colony-forming ability but γs-13 does not. For wt, this recovery is temperature-dependent with a maximum at 23°C, the most favorable growth temperature. Some recovery still occurs at 5-10°C, although growth does not. Strains wt and γs-18 are more sensitive when irradiated in oxygen or air compared with nitrogen. The sensitivity of γs-13 is the same in air, oxygen, or nitrogen. Postirradiation treatment with 1 mg caffeine/ml before plating sensitizes wt and γs-18 but increases the survival of γs-13. Strain γs-13 is more sensitive to killing by nitrosoguanidine than is wt or γs-18. These results suggest that γs-13 is more sensitive than wt because it is less capable of repairing gamma-ray and other damage and that γs-18 is intermediate in this ability.

Some effects of ultraviolet light on Blastocladiella emersonii have been investigated. An action spectrum obtained at the zoospore stage of development suggests a large nucleic acid involvement in lethality. Zoospores damaged by short-wavelength UV light could be photoreactivated. This result strongly suggests that the critical lesions are pyrimidine dimers, most likely in DNA. Studies of the previously observed cyclical variations in UV sensitivity were extended. Germlings at one of the more resistant stages of the life cycle (1.5 hours of development) did not photoreactivate as rapidly nor to the same extent as did the cells at the very sensitive zoospore stage. These results may indicate a differential repair process or a difference in photoproducts at these stages. Acriflavin and dose fractionation experiments were inconclusive in verifying the existence of repair. Caffeine produced a small differential effect in lowering UV survival at different stages of the life cycle, but cells grown in the presence of this chemical still exhibited a cyclical variation in sensitivity to UV light.

Populations of Blastocladiella emersonii zoospores can undergo synchronous germination, nuclear division, and development. Three characteristic responses have been noted for ultraviolet- or gamma-irradiated zoospores, namely: development to (1) a normal, surviving sporangium of about 60 microns in diameter with emission of zoospores and colony formation; (2) a giant sporangium of about 200 microns in diameter, sometimes giving off zoospores after a lag; and (3) a slower growing germling with the usual rhizoids, followed by an abrupt cessation of growth at a diameter of about 30 microns, with no zoospore emission. This "block" indicated in (3) occurs at about the same point with either UV or gamma rays over a considerable range of dose. During the earlier growth of this "blocked" germling, nuclear division does not occur normally. The UV damage is photoreactivable. Survival of colony-forming ability has been determined for germlings irradiated at various stages of their nuclear division up to the four nucleus stage. They are the most sensitive just after nuclear division and most resistant just before. Of several hypotheses to explain this variation, differential repair of damage seems to be the most plausible at present.

Experimental techniques are described whereby cultured mammalian cells have been irradiated with stripped ion beams of <tex-math>${}^{2}{\rm H},\ {}^{4}{\rm He},\ {}^{6}{\rm Li},\ {}^{7}{\rm Li},\ {}^{11}{\rm B},\ {}^{12}{\rm C},\ {}^{14}{\rm N},\ {}^{16}{\rm O},\ {}^{20}{\rm Ne}$</tex-math>, and <tex-math>${}^{40}{\rm Ar}$</tex-math>. The heavy ion linear accelerators at Berkeley and Yale have both been adapted to such studies, and the methods used in each of these laboratories are presented in this paper. In both cases scattering foils have been used to spread the beam to a useful size, and thin-walled ionization chambers have been used for the dosimetry. Precautions and corrections required for beam uniformity and accurate dose measurement have been established. One method allows the cells to be irradiated in a liquid environment, and the other in a gaseous environment. The ion velocity at the cells has in one case been identical for several different ions, and in other cases it has been different for each ion. The range of linear energy transfer represented by these ions extends from 65 to <tex-math>$19,500\ {\rm MeV}\text{-}{\rm cm}^{2}/{\rm gm}$</tex-math>. Some physical properties of accelerated <tex-math>${}^{40}{\rm Ar}$</tex-math> ions are described.