The goal of this study was to develop a small, stable liposomal carrier for antisense oligodeoxynucleotides (asODN) that would have high trapping efficiencies and long circulation times in vivo. Traditional cationic liposomes aggregate to large complexes and, when injected intravenously, rapidly accumulate in the liver and lung. We produced charge-neutralized liposome-asODN particles by optimizing the charge interaction between a cationic lipid and negatively charged asODN, followed by a procedure in which a layer of neutral lipids coated the exterior of the cationic lipid-asODN particle. The coated cationic liposomes had an average diameter of 188 nm and entrapped 85-95% of the asODN. The biodistribution and pharmacokinetics of an 18-mer 125I-labeled phosphorothioate ODN formulated by this method were determined after tail vein injection in mice. The majority of the asODN was cleared from blood, with a half-life of >10 hours compared with <1 hour for free asODN. When coupled with an anti-CD19 targeted antibody, this formulation was also effective at delivering an MDR1 asODN to a multidrug-resistant human B-lymphoma cell line in vitro, decreasing the activity of P-glycoprotein. No inhibition was found for nontargeted formulations or for free asODN. A number of therapeutic opportunities exist for the use of small, stable, long-circulating, and targetable liposomal carriers such as this, with high trapping efficiencies for asODN.