Dendrimers comprising a dentritic polypeptide with one dendron having terminal cationic groups and a lipidic anchor, preferably comprising C.sub.6-24-alkyl group containing .alpha.-amino acyl groups, preferably joined to the focal group, are used to assist transfection of cells in vitro and in vivo by DNA. The complex of dendrimer and DNA may be used in gene therapy, for instance to delivery clotting factor genes to cells.