The immunologic enhancement of Flavobacterium columnare exopolysacchar ide (EPS) was examined by semi-quantitative RT-PCR in grass carp (Ctenopharyngodon idellus) immunized with intraperitoneal injection of EPS. The total RNA was extracted from liver, spleen, trunk and head kidney of EPS and saline injected fish and reversetranscribed into cDNA, respectively. Five immune genes, i.e. CRP, IL-1, MHC-I, IgM and IFN were then amplified through polymerase chain reaction and semiquantified by comparing brightness of electrophoresis band of the gene PCR product with that of β-actin, respectively. The gene expression of CRP in liver of immunized fish increased significantly, and gene expression of MHC-I in spleen and trunk kidney of immunized fish decreased significantly in comparison with control fish. No significant change was observed in the gene expression of IL-1 between immunized and control fish, and the gene expression of IFN was weakly detected in tissues of both immunized and control fish with no significant difference observed. However, the gene expression of IgM in four tissues of immunized fish increased significantly when compared with control fish. It is thus indicated that the innate and specific humoral immunity of grass carp are effectively activated by immunization with EPS of F. columnare.