The aim of this study was to determine the changes in the survival, the haemocyte counts, malondialdehyde, superoxide dismutase, catalase, glutathione peroxidase, and phenoloxidase activities of larvae of Hyphantria cunea fed by diets containing various amounts of protein and carbohydrate contents and infected by Bacillus thuringiensis subsp. kurstaki. Second-generation larvae were used in the experiment. Six versions of artificial diets differing in the amounts of proteins and carbohydrates were used. In the infected groups, the number of the surviving larvae decreased compared to the uninfected groups. The malondialdehyde, the superoxide dismutase and the catalase activities increased as the amount of protein contents in the diets increased in both the uninfected and the infected groups. These results indicate that diet plays a significant role in the ability of H. cunea to have an effective immune response against the bacterial infections, with the availability of the protein content being the most important indicator of the functional (physiological) immune response and the carbohydrate contents being crucial to cellular immune function. As a result, it was found that the larvae exposed to the bacterial infection with diet imbalance tried to protect themselves with both cellular and antioxidant enzymes to combat these unfavourable conditions.