We have demonstrated that antisense phosphorothioate oligonucleotides (S-ODNs) inhibit influenza A virus replication in MDCK cells. Liposomally encapsulated and free antisense S-ODNs with four target sites (PB1, PB2, PA and NP genes) were tested for their abilities to inhibit virus-induced cytopathogenic effects in a MTT assay using MDCK cells. The liposomally encapsulated S-ODN complementary to the site around the PB2 AUG initiation codon showed highly inhibitory effects. In contrast, the inhibitory effect of the liposomally encapsulated S-ODN targeted to PB1 was considerably decreased in comparison with that directed to the PB2 target site. The liposomally encapsulated antisense S-ODNs exhibited higher inhibitory activities than the free oligonucleotides, and showed sequence-specific inhibition, whereas free antisense S-ODNs were observed to inhibit viral adsorption to MDCK cells. Liposomal preparations of oligonucleotides facilitated their release from endocytic vesicles, and thus cytoplasmic and nuclear localization was observed. The activities of the antisense S-ODNs were effectively enhanced by using the liposomal carrier. Interestingly, the liposomally encapsulated FITC-S-ODN-PB2-as accumulated in the nuclear region of MDCK cells. However, weak fluorescence was observed within the endosomes and the cytoplasm of MDCK cells treated with the free antisense S-ODNs. The cationic lipid particles may thus be a potentially useful delivery vehicle for oligonucleotide-based therapeutics and transgenes, appropriate for use in vitro or in vivo.