The present invention relates to new compositions for isolating and/or stabilising nucleic acids in materials of biological origin. The compositions comprise as an essential ingredient a cationic compound of general formula Y.sup.+R.sub.1R.sub.2R.sub.3R.sub.4X.sup.- wherein Y may represent nitrogen or phosphorus R.sub.1, R.sub.2, R.sub.3 and R.sub.4 independently of one another may represent a branched or unbranched C.sub.1-C.sub.20-alkyl group and/or a C.sub.6-C.sub.20-aryl group as well as a C.sub.6-C.sub.26-aralkyl group and X.sup.- may represent an anion of an inorganic or organic, mono- or polybasic acid.