We report here the enhanced adjuvant effect of murine interleukin-2 plasmid DNA (mIL-2) by complexation with a cationic polymer polyethylenimine (PEI). The mRNA expression of mIL-2 was observed at C2C12 cells after treatment with pVAXmIL-2 in PEI-complexed form, but not in naked plasmid DNA. In vivo, the plasmid DNA levels of mIL-2 at the administered muscle tissues were prolonged up to 14 days after intramuscular administration in PEI complexes. Moreover, the mRNA expression level in the muscle was 54-fold higher in PEI-complexed pVAXmIL-2 than in naked pVAXmIL-2 at 14 days after intramuscular administration. Mice were immunized on Days 0 and 28 by intramuscular administration of hepatitis B surface antigen (HBsAg) alone or with pVAXmIL-2 in naked or PEI complexes. At 8 weeks after the first immunization, mice coadministered with HBsAg plus pVAXmIL-2 in PEI complexes showed the serum IgG titer 72- and 208-fold higher as compared to those treated with HBsAg plus naked pVAXmIL-2 and with HBsAg alone, respectively. Though both HBsAg-specific IgG(1) and IgG(2a) immune responses were increased upon complexation of pVAXmIL-2 with PEI, IgG(2a) was more effectively induced relative to IgG(1). These results suggest that the complexation with PEI could be useful for prolonging in vivo expression of genetic cytokine adjuvants and enhancing their adjuvanticity for subunit vaccines.