Although it is well documented that the immunological activity of cytosine-guanine (CpG) motifs is abrogated by 5' methylation of the cytosine residue, encapsulation within stabilized lipid nanoparticles endows these methylated cytosine-guanine- (mCpG-) containing oligonucleotides (ODNs) with potent immunostimulatory activity in murine animal models. Surprisingly, not only do liposomal nanoparticulate (LN) mCpG ODN possess immunostimulatory activity, their potency is found to be equivalent and often greater than the equivalent unmethylated form, as judged by a number of ex vivo innate and adaptive immune parameters and anti-tumor efficacy in murine models. Preliminary data indicate that both methylated and unmethylated CpG ODN act through a common receptor signaling pathway, specifically via toll-like receptor (TLR) 9, based on observations of up-regulated TLR9 expression, induction of nitric oxide and dependence on endosomal maturation. This is confirmed in TLR9 knockout animals which show no immunostimulatory activity following treatment with LN-mCpG ODN. These data, therefore, indicate that the mCpG DNA is fully competent to interact with TLR9 to initiate potent immune responses. Furthermore, this work implicates an as yet unidentified mechanism upstream of TLR9 which regulates the relative activities of free methylated versus unmethylated CpG ODN that is effectively bypassed by particulate delivery of CpG ODN.