The present invention relates an additive comprising at least one substance selected from arginine, trometamol, meglumine, lysine, histidine, monoethanolamine, diethanolamine, triethanolamine, succinic acid, citric acid, tartaric acid, lactic acid, and salts thereof, which is useful in the prevention of aggregation of fine particulate complexes of a nucleic acid and collagen, and the production of a preparation comprising particulate complexes of controlled size which is suited for transporting a nucleic acid into cells.
