Abstract: The invention concerns a method for specifically eliminating at least one type of double strand DNA plasmid from a set of plasmids, characterised in that it consists in: preparing presynaptic filaments by fixing a protein with RecA activity on a single strand DNA whose sequence corresponds to a sequence of the plasmid to be eliminated and which further comprises a recognition sequence for a restriction enzyme sensitive to methylation; mixing the set of plasmid with the presynaptic filaments to form triplexes on the plasmids with the presynaptic filaments to form triplexes on the plasmids to be eliminated; methylating the resulting DNA mixture; releasing the triplexes; digesting the mixture obtained with the restriction methylation sensitive enzyme; if necessary, eliminating or separating the linear double strand DNA's corresponding to the plasmids to be eliminated. The method is useful for preparing DNAc libraries.