Intravenously injected plasmid DNA (pDNA) complexed with cationic liposome (lipoplexes) caused NF-kappa B-mediated cytokine production from macrophages, induced by CpG sequence in the pDNA. We have reported that cytokine production caused by linear polyethyleneimine (PEI)-pDNA complexes ( PEI polyplexes) was much lower than that caused by lipoplexes (Kawakami, S., Ito, Y., Charoensit, P., Yamashita, F., and Hashida, M. [ 2006]. J. Pharmacol. Exp. Ther. 317, 1382-1390). As Toll-like receptor-9 recognizing CpG sequence is expressed in the endosomal compartment, we hypothesized that the buffering capacity of PEI enhanced the escape of PEI polyplexes from endosomes, and that consequently cytokine production was decreased. In this study, the mechanism of lower cytokine production induced by PEI polyplexes, compared with lipoplexes, was investigated using the murine macrophage-like cell line RAW 264.7. Although transfection efficacy and cellular association were similar for PEI polyplexes and lipoplexes, tumor necrosis factor-alpha and interleukin-6 production and NF-kappa B activation caused by polyplexes were significantly lower than with lipoplexes. As for intracellular distribution, PEI polyplexes spread into cytosol whereas lipoplexes accumulated in vesicles, suggesting enhancement of escape from endosomes by PEI. Bafilomycin A1, an inhibitor of early endosomes, enhanced cytokine production and NF-kappa B activation by PEI polyplexes but not by lipoplexes; however, chloroquine, an inhibitor of late endosomes, inhibited PEI polyplex-induced cytokine production and NF-kappa B activation, suggesting that the buffering effect of PEI on early endosomes decreases NF-kappa B-mediated cytokine production. In conclusion, we demonstrate that cytokine production and NF-kappa B activation induced by PEI polyplexes are significantly lower than with lipoplexes in cultured macrophages. The significantly low cytokine response of PEI polyplexes may be due to effective transition of PEI polyplexes from endosomes to cytosol.