[en] Herpes simplex viruses and cytomegaloviruses were ⁶⁰Co γ-irradiated at -76⁰C and a dose-rate of 6.5 to 7.0 krad/min. The low irradiation temperature was chosen to minimize damage to surface antigens by radiation-induced radicals. Comparisons were made of the survival of propagative ability and the antigenicity of irradiated virus. The survival curves were consistent with single-hit exponential inactivation kinetics, and a virus-specific immune response was induced after a single immunization with completely inactivated virus. The titre of antibody reached was comparable to that expected after a single inoculation of viable virus and the inactivated virus could function as a stimulating antigen in a lymphocyte transformation assay. The results indicate that the antigenic components on individual virions remain largely intact after irradiation. (U.K.)