Industry testing of raw milk is usually only undertaken for the presence of mesophilic and thermoduric microorganisms. Hydrolytic enzymes, such as proteases and lipases produced by microorganisms, are rarely assayed because of the unavailability of test procedures with sufficient sensitivity to detect trace levels of these enzymes. A range of assays has been developed for detection of protease including those based on chromogenic and fluorogenic substrates. Examples of chromogenic substrates are hide powder azure (Cliffe and Law 1982), azocoll (Chavira Jr et al. 1984) and azocasein (Ewings et al. 1984) while fluorogenic substrates include 4-methylumbelliferyl casein (Khalfan et al. 1983) and fluorescein isothiocyanate-casein (FITC-casein) (Twining 1984). A common application of these assays has been as rapid assays (incubation times less than 1 hour) with either pure protease or samples containing high concentrations of protease. However, in that format, they are not suitable for detecting the very lowlevels of protease that can result in spoilage of UHTmilk after 6–9 months of ambient storage. Therefore, the aim of this study was to increase the sensitivity of existing assays, through various modifications, so that they could be used to detect very low levels of protease in UHT milk.