Based on the above results, the FITC-casein assay, when used according to the method of Christen and Senica (1987), but modified to include a longer incubation time, of up to a few days, is suitable for detecting very low levels of protease in UHT milk. The assay is highly sensitive, with acceptable stability of the substrate. A protease assay with casein as substrate, buffered to pH 7.0 with 100 mM Tris-HCl is a more cost-effective alternative. Its main drawback compared with the FITC-casein assay is its lower sensitivity. This means that even longer assay incubation times (longer than 14 days) would be required for detection of very low protease levels with the casein assay. Furthermore, both assays have the potential to be developed as assays for detection of plasmin, the native milk protease, also responsible for a range of quality deterioration issues.