The shelf-life of a culture medium is the maximum period of validity for optimum preparation and preservation. Apart from the composition of the medium, the factors that influence shelf-life are sterilisation method, preservation and packaging procedures, storage temperature and exposure to light. The shelf-life of a culture medium is defined by evaluating its basic chemico-physical characteristics so as to obtain the correct growth and characterisation of a specific microorganism. This research was conducted from March to September 2003 on 12 'critical' culture media, i.e. media that had a coded shelf-life of not more than thirty days. Each medium was produced in three separate batches, with a total of 5 940 samples. The purpose of the study was to define a longer period of validity than that coded for each medium by evaluating weight reduction, pH, fertility and sterility. The shelf-life observed for each medium was longer than those coded. The new shelf-life takes into account both the operational needs of complex organisational structures and the efficiency of the medium, depending on its chemico-physical characteristics and storage and preservation methods.