Background : Our previous studies showed that a plasmid encoding the Flt3 ligand cDNA (pFL) as a nasal adjuvant enhanced both mucosal and systemic antibody (Ab) responses, mediated by IL-4 producing CD 4+ T cells. On the other hand, synthetic oligodeoxynucleotides (ODN) containing one or more unmethylated cytosine-guanine dinucleotide (CpG) motifs have mucosal adjuvant activity associated with Th1-type cytokine responses. In this study, we examined whether pFL given with CpG as a mucosal adjuvant might elicit balanced Th1- and Th2-mediated antigen (Ag) -specific Ab responses. Methods : BALB/c mice were nasally immunized once a week for three consecutive weeks with 100μg of ovalbumin (OVA) plus 10μg of CpG ODN, 50μg of pFL, or a combination of 50μg of pFL and 10μg of CpG ODN. Results : The mice immunized with OVA together with both pFL and CpG ODN showed significantly higher levels of OVA-specific plasma IgG Ab and secretory IgA (S-IgA) Ab in the saliva as compared with mice immunized with CpG ODN only. Importantly, the OVA-specific plasma IgG and S-IgA Ab responses induced by the combination of pFL and CpG ODN as nasal adjuvants persisted for more than four months after the final immunization. Furthermore, plasma levels of IgA Ab also continuously increased during this period. Conclusions : These results suggest that pFL and CpG ODN represent an effective mucosal adjuvant combination. This work was supported by NIH grants and contracts DE 12242, AI I8958, AI 43197, and DC 04976.