Complexes (lipoplexes) between cationic liposomes and single-strand oligodeoxynucleotides (ODN) are potential delivery systems for antisense therapy. The nanometer-scale morphology of these assemblies is relevant to their transfection efficiency. In this work the monocationic lipid dioleoyloxytrimethylammoniumpropane, the neutral "helper" lipid cholesterol, and an 18-mer anti-bcl2 ODN were combined at different ratios. The lipoplexes formed were characterized for the quantity of ODN bound, for the degree of lipid mixing, and for their size. The nanostructure of the system was examined by cryogenic-temperature transmission electron microscopy, augmented by small-angle x-ray scattering. Addition of ODN to cationic liposomes induced both liposome aggregation and the formation of a novel condensed lamellar phase. This phase is proposed to be stabilized by anionic single-strand ODN molecules intercalated between cationic bilayers. The proportion of cholesterol present apparently did not affect the nature of lipoplex microstructure, but changed the interlamellar spacing.