Abstract: The present invention is concerned with modified poly-(propylene imine) dendrimers, comprising cationic internal ammonium groups and external non-toxic endgroups, pharmaceutical compositions comprising said dendrimers, methods for the production of said dendrimers and their use as transfections agents for anionic bioactive therapeutic factors, for use in gene therapy, in particular for the treatment of cancer. The modified poly-(propylene imine) dendrimer of generation 1, 2, 3, 4 or 5, also comprising incomplete dendrimers and mixtures thereof, comprising external end groups and internal amine functions are characterized in that: (a) substantially all external endgroups are groups of formula (I) ?wherein R is a radical selected from the group of C1-10alkyl, polyethylene glycol radical and polyethylene glycol gallyl radical; and (b) substantially all internal amine functions are quaternary cationic ammonium functions.