The present invention relates to a method of isolating a desired nucleic acid from a biological solution, which method comprises to selectively precipitate the desired nucleic acid by adding a polycationic precipitating agent to the solution and allowing it to form a complex with said nucleic acid, wherein the precipitating agent is a highly charged linear polymer that comprises quaternary amino groups. The polycationic precipitating agent is preferably added in such an amount that the charge ratio [+]/[-] between polycationic precipitating agent and nucleic acid is .gtoreq.0.5, preferably .gtoreq.0.9 and most preferably .gtoreq.1 during the precipitation, and in the presence of a salt concentration ensuring the quantitative specific precipitation of the nucleic acid/polycation complex.