Abstract: The invention is related to improved nucleic acid reagents comprising arrays of nucleic acid fragments and combinations of such fragments. The preparation of such fragments by recombinant DNA techniques and their use in sandwich hybridization methods is also described. By making different combinations of the nucleic acid fragments--some labeled and some affixed to solid carriers, it is possible to create kits for the identification of e.g. veneral diseases.The improved nucleic acid reagents comprise two series of at least two alternating nucleic acid fragments, which are homologous to sequences in the nucleic acid to be identified, one of the series being labeled and one affixed to a solid carrier. Nucleic acid fragments belonging to different series must not be homologous to each other.Sandwich hybridization tests performed with arrays of nucleic acid fragments are at least four times as sensitive as sandwich hybridization tests performed with reagents belonging to the prior art.