Nuclease digestion and intracellular delivery are major factors limiting the potential use of oligodeoxynucleotides as antisense molecules. Structural analogues of phosphodiester oligodeoxynucleotides, such as phosphorothioates and methylphosphonates, are resistant to nuclease degradation and can still bind to their mRNA targets. However, their limited ability to escape from the endosomal/lysosomal compartments and reach the intracellular sites of action have dampened their potential clinical application. To circumvent this problem we have incorporated methylphosphonate oligodeoxynucleotides into liposomes. We found that the level of uptake of liposome-incorporated methylphosphonate oligodeoxynucleotides is time and concentration dependent. Maximal up take occurred at 8 h when 4-8 microM liposome-incorporated methylphosphonate oligodeoxynucleotides was added. Approximately 50% of liposome-incorporated methylphosphonate oligodeoxynucleotides were retained in cells after 24 h of incubation. Using fluorescent microscopy, intracellular fluorescence could be seen within 2.5 h of incubation. Diffused fluorescence was found throughout the cytoplasm, suggesting that the liposome-incorporated methylphosphonate oligodeoxynucleotides were not confined within the endosomal/lysosomal structures. We conclude that liposomes can effectively deliver methylphosphonate oligodeoxynucleotides to the cytoplasm, which is the major intracellular site of action for translational arrest.