To investigate the immunogenicity of Bac-RSV/G, the groups of mice were immunized i.n. with Bac-RSV/G, Bac-control, or PBS. RSV G-specific antibody levels in sera from the immune mice at 2 weeks after priming and 2 weeks after boost-ing were determined by ELISA. The specific antibody responses were barely detectable in all groups of mice at 14 days after priming (data not shown). Following booster immunization, however, the mean titers of serum antibodies increased significantly only in the group of mice immunized with Bac-RSV/G ( Fig. 2A) . Secretory IgA has been shown to directly mediate local immunity against aerial pathogens, implying an important role for antibody in protection in the upper respiratory tract (22) . Thus, for an effective RSV vaccine development, the induction of secretory IgA on respiratory mucosal surface is critical. To examine whether Bac-RSV/G vaccination elicits IgA response in the respiratory tract, bronchoalveolar lavage (BAL) was performed at day 5 after RSV challenge and levels of IgA were determined by RSV-specific ELISA. Each group of mice immunized with Bac-control or PBS did not exhibit any specific IgA response, whereas Bac-RSV/G-immune mice showed significantly enhanced level of specific IgA response (Fig. 2B) . These results suggest that intranasal immunization of Bac-RSV/G effectively induce both RSV G-specific serum IgG and respiratory IgA.