Abstract: This study was performed to estimate the hypothesis that the major outer membrane(MOMP) of Aeromonas hydrophila is possibly the important common protective antigen to fish1MOMP of four strains Aeromonashydrophila which belong to three different serogroups were prepared by treatment with 1. 2% N-lauroylsarcosinate detergent, and then concentrated and purified by utracentrifuging1Structural analysis exhibited that threeof them havethe sameMOMP pattern and themajor protein bands were 40 kilodalton and 43-kilodalton in SDS-PAGE gel1Strain WC-2 has another MOMP pattern To enhance the immunogenicity of MOMP,MOMP-ISCOMs(Immune-stimulating complex, ISCOMs)were prepared by dialysis assay Transmission electron micrograph showed that MOMP-ISCOMs prepared in this study had some basicmatrix profiles contrast to the typical ISCOMs European eels(Anguillia anguillia)were immunized by intraperitoneal injection(i1p)with a dose of 20Lg per fish with these MOMP-ISCOMs1Serum were collected on fourteenth day and thirtieth day, respectively.The average serum antibody titers ranged from 1B320) 1B1280 by enzyme linked immunosorbent assay (ELISA)1Immunoblot indicated that all these four MOMP and four MOMP-ISCOMs could be completely recognized by the corresponding anti serum Challenge experiment was carried out on fortieth day with the virulent strain TPS-30 whose 50% ethal dosage(LD50) to eels(20-30g per fish) is about 3.0 106CFU(clony forming unit) per fish. No protective effect were observed when challenged with a dose of 3.0 108CFU/fish, but over 80% relative percent survival(RPS)were observed when given a challenge with a dose of 3.0 107CFU/fish.All these results showed that MOMP of A1hydrophila used as ISCOMs form is important protective antigen. A low immunized dosage could stimulate eels to produce high protective immunity, and no distinct difference was observed among different O-serogroups strains