We found that the intramuscular injection of vitamin B₂ enhanced host resistance to E. coli infection in a dose-dependent manner (6.25 mg/kg-100 mg/kg). Furthermore, VB₂ exhibited the protective activity against Pseudomonas aeruginosa, Klebsiella pneumoniae, Staphylococcus aureus, and Actinobacillus pleuropneurnoniae. The mechanism of action of VB₂ for enhancing resistance in mice may be, at least in part, its ability to stimulate the multiplication of neutrophils and monocytes, and to activate macrophages.