To better understand the structures formed by the interaction of cationic lipids with DNA, we undertook a systematic analysis to determine the biophysical characteristics of cationic lipid:DNA complexes. Four model cationic lipids with different net cationic charge were found to interact in similar ways with DNA when that interaction was compared in terms of the apparent molar charge ratio of lipid to DNA. When DNA was present in charge excess over the cationic lipid, the complex carried a net negative charge as determined by zeta potential measurements. Under these conditions, some DNA was accessible to ethidium bromide, and free DNA was observed in agarose gels and in dextran density gradients. Between a lipid:DNA charge ratio of 1.25 and 1.5:1, all the DNA became complexed to cationic lipid, as evidenced by its inaccessibility to EtBr and its complete association with lipid upon agarose gel electrophoresis and density gradient separations. These complexes carried a net positive charge. The transition between negatively and positively charged complexes a occurred over a very small range of lipid to DNA ratios. Employing a fluorescent lipid probe, the addition of DNA was shown to induce lipid mixing between cationic lipid-containing vesicles. The extent of DNA-induced lipid mixing reached a maximum at a charge ratio of about 1.5:1, the point at which all the DNA was involved in a complex and the complex became positively charged. Together with freeze-fracture electron micrographs of the complexes, these biophysical data have been interpreted in light of the existing models of cationic lipid:DNA complexes.