A process for the isolation and purification of nucleic acids and/or oligonucleotides for use in gene therapy wherein said nucleic acids and/or oligonucleotides are isolated or purified from an essentially biological source, characterized in that said essentially biological sources are lysed, the fractions obtained are optionally freed or depleted from the remainder of said biological sources by per se known mechanical methods, such as centrifugation, filtration; the fractions thus treated are subsequently treated with affinity chromatographic material or with inorganic chromatographic material for the removal of endotoxins; followed by isolation of said nucleic acids and/or oligonucleotides on an anion exchanger which is designed such that DNA begins to desorb from the anion exchanger only at an ionic strength corresponding to a sodium chloride solution of a concentration higher by at least 100 mM than one corresponding to the ionic strength at which RNA begins to desorb from the anion exchanger material.