The endosomal escape of the polyion complex between DNA and polycation is one of important factors for efficient gene expression. Here we have examined whether the poly(L-histidine) with several aminoethyl groups, i.e., aminated poly(L-histidine), possessed the pH-sensitive properties as DNA carrier. The polyion complex formation of the aminated poly(L-histidine) with DNA was affected by the endosomal pH of the medium, leading to significant transfection activity.