A novel cationic cholesterol derivative, 3beta[N-(N',N',N'-trimethylaminopropane)-carbamoyl] cholesterol iodide (Chol-Q), has been formulated with equimolar amounts of dioleoyl phosphatidylethanolamine (DOPE) into stable unilamellar liposomes up to 100 nm in size for DNA delivery into mammalian cells. When compared with similarly constituted liposomes containing the tertiary analogue 3beta[N-(N',N'-dimethylaminopropane)-carbamoyl] cholesterol (Chol-T) in a band shift assay, liposomes displayed similar DNA binding affinities and appeared to afford complete protection to plasmid DNA against serum nuclease catalysed degradation at liposome:DNA ratios (w/w) of 2.5:1, 5:1, and 10:1 in incubation mixtures containing 5% fetal bovine serum at 37 C for 90 min. Chol-Q liposomes were, however, markedly less toxic to cells in culture over a wide range of concentrations with cells numbering 76% of untreated controls at 37.5 microg/mL complete medium in the human hepatocellular carcinoma line HepG2 and 75% at 30 microg/mL in cervical carcinoma HeLa cells. At these levels of Chol-T liposomes, cell numbers were 37% and 15%, respectively. Gene transfer experiments with pSV2CAT and pRSVCAT plasmids in HepG2 cells showed maximum efficiency at a Chol-Q liposome:DNA ratio of 5:1 (w/w) and at a Chol-T liposome:DNA ratio of 10:1. In HeLa cells, both liposome preparations performed best at a ratio of 2.5:1. Differences in transfection efficiencies over the liposome range of 5-20 microg/ mL were rather less pronounced with Chol-Q lipoplexes suggesting a greater versatility of this system.