The invention relates to a composition for use as a stabilizer and a method for stabilizing biological samples, in particular of whole blood, in particular for stabilizing the content and the integrity of cell-free nucleic acids. The composition has, in aqueous solution, at least one buffer substance, which buffers to a pH value of 7 or less, preferably 3.5 to 7.0, at least one chelating agent for divalent cations and urotropine, optionally PEG, or consists thereof.
