Sharon, Nehama (University of Notre Dame, Notre Dame, Ind.), Morris Pollard, and W. R. Klemm. Psittacosis-infected cell system for studies on radiation damage. J. Bacteriol. 86:1139-1144. 1963.-The unique replication pattern of the agent of psittacosis in cells provides a means for studying biological effects of radiation and the influence of radioprotective drugs on this cell virus system. Intracellular psittacosis virus (strain TT) showed a distinctive sensitivity to X irradiation at 3 to 9 hr after onset of infection. This sensitivity to X rays disappeared later in the replication cycle when the virus-orientated ribonucleic acid matrix appeared in the infected cell. The effect of X rays was on the early stages of the virus, since evidence of subsequent developmental stages could not be observed. During the sensitive stage, the amount of surviving virus was related to the dose of X rays. Irradiation caused an irreversible reaction on sensitive virus. The progeny of surviving virus were unaltered in susceptibility to X rays. Addition of glutathione to infected cultures before irradiation increased the amount of surviving virus. Cysteine.HCl and heat-inactivated calf serum were only protective to the virus. Cysteamine was toxic to the cells, and low concentrations gave no protection. Other compounds tested failed to protect the viral agent against the effect of X rays.