Melittin-polyethylenimine (PEI) conjugates have been shown to enhance gene transfer efficiency of polyplexes due to their membrane-destabilizing properties. Inherent lytic activity at neutral pH however also provokes high cytotoxicity due to plasma membrane damage. In order to shift the lytic activity towards the endosomal membrane, several melittin analogs were designed. Acidic modification of melittin by replacing neutral glutamines (Gln-25 and Gln-26) with glutamic acid residues greatly improved the lytic activity of C-terminally linked PEI conjugates at the endosomal pH of 5. This activity correlated well with the gene transfer efficiency of polyplexes in four different cell lines. Melittin-PEI conjugates with high lytic activities at endosomal pH were then incorporated into EGF receptor-targeted and polyethylene glycol-shielded polyplexes. The resulting particles had virus-like dimension (150 nm) with a neutral surface charge and were subsequently purified by size exclusion chromatography to remove unbound toxic PEI conjugate. These purified polyplexes mediated EGF-receptor-specific gene transfer with up to 70-fold higher activity compared to the corresponding PEI polyplexes without melittin.