This study investigated the efficiency of intrahaemocoelic injection of sublethal concentration (LC₂₀ = 5.7×10³ cells/ml) of Salmonella typhimurium in the induction of immune response in 5^th larval instar of Spodoptera littoralis. The role of antimicrobial peptides (AMPs) induced in the larval hemolymph was examined as natural antibiotics against different Gram-negative (G-ve) or Gram-positive (G+ve) bacteria. Where the larval plasma injected with Salmonella showed a significant antimicrobial activity against different strains of pathogenic bacteria (Staphylococcus aureus, Salmonella typhimurium, Escherichia coli and Enterococcus faecalis). The biochemical characterization of the immune hemolymph indicated drastic changes in both the total protein content and the protein banding patterns following bacterial injection. The total hemolymph proteins (THPs)decreased significantly 48 h post-Salmonella-injection. The hemolymph proteins profile was also qualitatively changed through the induction of new proteins and the disappearance of others simultaneously, which may be attributed to their participation in the immune reactions. The appearance of six new synthesized protein bands that were found to be specific for Salmonella injection, with a characteristic band of MW ~22 KDa predicting that this protein band may be Attacin-like AMP.