The DN-ase test is the most important tests used for identification of pathogenic microorganisms. But often this test is not accessible for microbiology laboratories because of its high prices. The author elaborated a new original preparation for the rapid determination of the DN-ase activity, which permits a 350-400 fold reduction of the cost/test ratio. Moreover, the technique procedure is essentially simplified; there is not necessary to cultivate the microbial strain in nutritional media before testing and thus the necessity of laboratory sterile materials is also diminished. These advantages were achieved by using Ustilago maydis chlamidospores, ("bisorbent MN-21") as laboratory reagent for DN-ase activity.