The variations in the amounts of milli-calpain and its specific inhibitor in the white muscle of sea bass (Dicentrarchus labrax) during storage at 4 degreesC for up to 7 days were determined after separation by hydrophobic chromatography on a Phenyl Sepharose vel. There was a significant decline in post-slaughter m-calpain activity with an important inter-individual variability in the rate of decrease of the total activity. In contrast with the calpastatin of mammalian post mortem muscles, calpastatin remained constant within fish muscles after death. The initial levels of protease and inhibitor activities, and their behaviour through post mortem storage. are discussed and implications for the mechanism of tenderisation of fish muscle are suggested. (C) 2003 Elsevier Ltd. All rights reserved.