Crude proteases from P. fluorescens isolates 65 and 117 were highly active in the casein and SMP assays; however, crude protease from P. fragi isolate 102 produced a significant response (P < 0.05) with SMP as substrate only when the 3-M buffer at pH 7.0 was used. For the other two isolates, the highest degree of protease action was observed with the 100-mM pH 7.0 buffer for both casein and SMP substrates, with casein being preferred over SMP (P < 0.05) (Table 1). In the SMP assays, the action of protease was considerably lower (P < 0.05) in the 3-M buffer, which was required to achieve an assay pH of 8.0. The clear preference for casein as substrate was expected as proteases from psychrotrophic pseudomonads show a distinct preference for casein as substrate over whey proteins (Mitchell and Ewings 1985). In the present study, the protein concentration was the same in both casein and skim milk assay mixtures, but the casein assay mixture contained a higher concentration of preferred proteins. Synthetic substrates such as azocasein and FITCcasein are typically used in protease assays with short incubation times. Therefore, optimization of the assay mixture composition and assay conditions was required to make them suitable for longer incubations, necessary to detect very low levels of protease. The azocasein assay is a simple and straightforward colorimetric protease assay. The results of the present study indicate that azocasein could be a suitable substrate for detection of low protease levels in UHT milk. There was only a negligible decomposition of the azo dye over the 24-h incubation period of the assay. The addition of crude protease to the UHT milk samples resulted in quantifiable proteolysis for each concentration of each crude enzyme preparation (Table 2). Furthermore, with P. fluorescens isolates 65 and 117, an increase in the concentration of added crude protease resulted in an increase in release of the dye from the substrate (P < 0.05). Assays based on fluorogenic substrates are generally regarded as more sensitive than those based on chromogenic substrates, and this was true for FITC-casein compared to azocasein (Table 2). An increase in the concentration of added crude protease resulted in an increase in fluorescence intensity for P. fluorescens isolates 65 and 117. In addition, there was no substrate decomposition over the long incubation time employed.