Chickens from lines selectively bred for either a high-antibody (HA) or low-antibody (LA) response to sheep erythrocytes were injected intravenously with Mycobacterium avium while being held in low, medium, or high levels of social stress for 5 days (first environment). During the remaining 6 weeks, they were held under either low or medium levels of social stress (second environment). Infection led to lesions consisting of granulomas, some of which had necrotic centers. There was a positive correlation between numbers of lesions with necrotic centers and M. avium cells recovered from livers. The numbers and nature of lesions were influenced by both genetic and environmental factors. Numbers of necrotizing lesions increased with stressfulness of the first environment. Total numbers of lesions were reduced by the medium-stress second environment, and the total number of necrotizing lesions was reduced among LA chickens in the low-stress second environment.