HBsAg-specific immunoglobulin G (IgG) isotype responses after intranasal (IN) immunization using small volume. BALB/c mice (n = 5–10) were immunized by IN inhalation of 10 µg of HBsAg alone (none) or with 1, 10, or 50 µg cholera toxin (CT) and/or CpG-containing oligode-oxynucleotides (CpG) as adjuvants in a volume of 20 µl. Mice were boosted at 4 and 8 weeks. Each bar represents the group geometric mean (± standard error of the mean; SEM) of the ELISA endpoint dilution titer for HBsAg-specific antibodies (anti-HBs geometric mean titers; GMT) of IgG1 (gray bars) or IgG2a (black bars) isotypes in plasma in plasma taken 4 weeks after the third and final immunization. Titers were defined as the highest plasma dilution resulting in an absorbance value two times that of non-immune plasma, with a cut-off value of 0.05. HBsAg, purified hepatitis B surface antigen.