A simple fluorescence assay for liposome stability is described. A fluorogenic enzyme substrate. 4-methylumbelliferyl-β-D-glucuronide, is encapsulated in liposomes. In the presence ofE. Coliβ-glucuronidase, any of the substrate that has been released into the medium is converted to 4-methylum-belliferone, which is monitored fluorimetrically. The method offers several advantages over current methods, including high gain, extremely low leakage, and facile quantitation. The use of the assay as part of a prototype immunoassay is decribed.