The interaction between T2, T4, T5, T7, and lambda bacteriophages double stranded DNA and cationic liposomes made up of one single synthetic cationic lipid, dioctadecyldimethylammonium bromide (DODAB), is quantitatively evaluated from a physicochemical point of view. The first step of the interaction is driven by the electrostatic attraction between DNA and bilayer; with probes being displaced from their DNA or bilayer sites. Under conditions of DODAB excess, at maximal DODAB adsorption on DNA, there are ca 70 DODAB molecules adsorbed per nucleotide on DNA, a molar proportion (MP) that does not depend on DNA type. Above charge neutralization, there is DNA -induced liposomal rupture, as evaluated from dialysis of DNA/liposome mixtures where liposomes contain [C-14]-sucrose in their internal compartment. In water, this DNA-induced leakage of radioactive liposomal contents suggests that the interaction DNA/cationic bilayer is not superficial. The DODAB/DNA interaction led to formation of globules as visualized from dark-field optical microscopy and to occurrence of a linear dependence between turbidity for the mixture and 1/lambda(2). At maximal DODAB adsorption, the formation of DODAB/DNA globular complexes causes loss of double-stranded DNA hypochromism as detected from temperature effects on DNA absorbance at 260 nm in the presence or absence of DODAB. In summary, the DODAB/DNA interaction is not at all superficial as expected merely from the electrostatic attraction between oppositely charged molecules: liposome loses its integrity and DNA loses its double helix becoming single-stranded. The hydrophobic attraction between nitrogenous bases on DNA and hydrocarbon chains on liposome bilayers plays an important role in determining the new physicochemical properties of the complex.