Mikrobiološka kontaminacija hranjivih podloga koje se upotrebljavaju za uzgajanje mikroorganizama prilično je česta u mikrobiološkim laboratorijima. Tijekom pripreme hranjivih podloga za uzgoj i izolaciju čistih bakterijskih kultura vjerojatnost kontaminacije velika je ili zbog neadekvatne sterilizacije, nepravilnog rukovanja ili neadekvatne kontrole uvjeta pohrane i provjere roka trajanja. Pripremljene ploče ne smiju se odmah upotrebljavati ili čuvati na niskoj temperaturi dok se ne testiraju na sterilnost. Provjera sterilnosti vrlo je važan segment prije upotrebe ploča za mikrobiološke postupke i procedure i jedan od glavnih faktora koji utječe na validnost rezultata ispitivanja u mikrobiološkom laboratoriju. Cilj je rada uspostaviti kriterije odbacivanja kontaminiranih ploča. Pripremljena su tri seta ploča hranjivih podloga koji su tijekom 18 do 24 sata inkubirani na sobnoj temperaturi za provjeru sterilnosti. Procjena kontaminacije: porast kolonije na površini ili ispod nje. Inkubacija ploča produžena je za još jedan dan za razvoj kontaminacije na pločama na kojima nije bilo porasta kolonija prvog dana. Rezultati su otkrili da su u dvije skupine pronađene dvije vrste kontaminacije. Prvi set ploča ima kolonije na površini i kontaminacija nije pronađena drugog dana na nekontaminiranim pločama, što ukazuje na to da bi se kontaminacija mogla dogoditi tijekom postupaka nepravilnog rukovanja pločama. Druga skupina ima kolonije ispod površine i zagađenje se drugog dana razvijalo i u nekontaminiranim pločama zbog nepravilne sterilizacije staklenog posuđa ili nepravilne sterilizacije hranjivih medija.; Microbial contamination of the nutrient media used for the cultivation of microorganisms is quite common in microbiology laboratories. During the preparation of nutrient media for the cultivation and isolation of pure bacterial cultures, the likelihood of contamination is high either due to inadequate sterilization, improper handling or inadequate control of storage conditions and to check shelf life. Prepared plates should not be used immediately or stored at low temperature until they are tested for sterility. Sterility testing is a very important segment prior to the use of microbial plates and procedures and one of the major factors affecting the validity of test results in a microbiology laboratory. The aim of the paper is to establish criteria for rejection of contaminated plates. Three sets of plates of nutrient medium were prepared and incubated at room temperature for 18-24 h to check for sterility. Contamination assessment: colony growth on or below the surface. Plate incubation was extended by another day for the development of contamination on plates on which no colonies had grown on day one. The results revealed that two types of contamination were found in the two groups. The first set of plates had colonies on the surface and contamination was not found the next day on uncontaminated plates indicating that contamination could occur during improper handling of the plates. The second group has colonies below the surface and pollution developed on the second day in non-contaminated plates due to improper sterilization of glassware or improper sterilization of nutrient media. Understanding the importance of sterility testing before using nutrient media for the cultivation of microorganisms of interest, it is recommended that sterility confirmed plates be used for further microbiological procedures, while all other sets of contaminated plates establish rejection criteria regardless of how they are produced and what type of contamination is found.