This study was performed to determine the degree to which 60 Co- n irradiation alters the susceptibility of conventional mice to intragastric challenge with Shigella sonnei as measured by survival, body weight, performance, and microbiology assays of intestinal flora. Female B6D2F1/J mice were given S. sonnei intragastrically 4 days after sublethal 60 Co- n irradiation. Survival and body weight after irradiation and pathogen challenge and under control conditions were examined for 30 days while motor coordination, using the inverted-screen test, was examined intermittently during the 30-day period. Survival and weight loss in irradiated and challenged mice depended on the combined doses of the two insults. The incidence of mortality in the latter mice was never 100%, even at a high sublethal radiation dose (7-Gy) and a S. sonnei dose of 1.2 2 10 9 CFU. Depending on the combined doses of the two insults, weight loss in surviving mice was generally recovered within 30 days. 1.2 2 10 8 CFU S. sonnei evoked mortality in four of 12 non-irradiated but challenged mice (33%) but in no other case did S. sonnei alone evoke mortality. Radiation alone evoked no mortality. Radiation alone significantly reduced body weight when mice were individually weighed on a daily basis. Performance on the inverted-screen test was significantly decremented in irradiated, challenged mice. S. sonnei was always recovered from the heart blood of deceased irradiated-challenged mice but was variably recovered from ileum or caecum and seldom recovered from formed, expressed stools. We conclude that conventional mice can be made susceptible to an intragastric challenge of S. sonnei by sublethal doses of ionizing radiation.