Mice were given intraperitoneal injections of the radioprotective drug,n-decylaminoethane thiosulfuric acid (n-C10H21-NH-(CH2)2-S2O3H), immediately prior to whole body X-irradiation at doses varying from sublethal (100R) to lethal (840R). In order to test the effect of this drug on the susceptibility of mice to infection, an attenuated living bacterial vaccine,Pasteurella tularensis(living vaccine strain, LVS), was injected subcutaneously at times varying from one hour to 28 days after irradiation.n-Decylaminoethane thiosulfuric acid was 100% effective in protecting non-infected mice against lethal irradiation and did not have any observable effect on non-irradiated mice infected withP. tularensis(LVS). However, if protected-irradiated mice were infected earlier than the tenth day post-irradiation, 100% deaths resulted. Cultures from autopsied mice showed large numbers of the bacterium in the liver, spleen, lymph nodes and blood. The susceptibility of protected-irradiated mice to infection decreased as the time after irradiation increased. Thus, it appeared that a bacterial infection from a living vaccine given earlier than one week after irradiation may abolish or mask the protective effects of the drug.