After incubation of four of ten strains of multi-drug resistant Pseudomonas aeruginosa (MDRP) with the healthy human serum for one hour, the bacteria lysed and the viable cell count decreased with the disappearance of the C3 complement. Over 99% of IgG was deleted from the serum by passing the serum through a Protein G Sepharose column. In such IgG-deleted serum, the bactericidal activity on P. aeruginosa was cancelled. The bactericidal activity recovered by adding human intravenous immunoglobulin preparation (IVIG) at the concentration of 2-4 mg/mL to the serum. Moreover, a strong bactericidal activity was shown in the case of some strains that the antibody titers of IVIG were high. It was thought that specific antibodies to some of P. aeruginosa are contained in IVIG and activated the classical pathway of complement system; as a result, the bacterial lyses had been caused. Sera from healthy persons contain antibodies to P. aeruginosa as well as complement components. Since the complement-mediated bactericidal activity disappeared by removing IgG in this study. These results suggest that the IVIG therapy in bacterial infections is expected to recover the immune function so as complement-mediated bactericidal activity as well as opsonic activity against P. aeruginosa, especially in the patient with neutropenia.