Cationic liposomes and the complexes they form with DNA (lipoplexes) constitute the most promising alternative to the use of viral vectors for gene therapy. One of the limitations to their application in vivo, however, is the inhibition of gene delivery by serum. In a previous study, we demonstrated that transferrin (Tf)-lipoplexes were superior to plain lipoplexes in transfecting HeLa cells in the presence of high concentrations of serum. With the goal of obtaining efficient gene expression in vivo, we evaluated the efficacy of Tf-lipoplexes (containing DOTAP and cholesterol) in transfecting primary hepatocytes and adipocytes in the presence of high serum concentrations. The association of transferrin with cationic liposomes increased luciferase expression compared to plain lipoplexes in primary cells as well as in HepG2 and 3T3-L1 differentiated adipocytes. The complexes were not cytotoxic and were highly effective in protecting DNA from attack by DNase I. An efficient and reliable method was developed to prepare lipoplexes containing both Tf and protamine sulfate, where the latter was mixed with transferrin, followed by the addition of cationic liposomes and DNA. The resulting protamine-Tf-lipoplexes increased significantly the levels of gene expression in cultured cells and in various tissues in mice following i.v. administration.