Longissimus thoracis (LT) of 10 Blonde d'Aquitaine young bulls were sampled at slaughter. Protein composition of fresh muscle and of meat aged for 14 days was investigated by two-dimensional electrophoresis. Cooked meat properties were also evaluated by sensory analysis. When searching for early predictors of tenderness, abundance of succinate dehydrogenase (SDH) was the best common predictor of initial and overall tenderness, explaining 65.6% and 57.8% of variation of these palatability traits. Study of the evolution of the protein content during ageing allowed to identify targets of postmortem proteolysis. They were mainly structural (actin, MyBPH) and chaperone (HSP27, alpha-crystallin) proteins. Furthermore, in a regression analysis modelling sensory tenderness, levels of HSP27 in fresh muscle and levels of HSP27 fragments in aged meat explained up to 91% of variation in sensory scores. Data suggest the existence of an underlying HSP27-related cellular mechanism, with consequences on tenderness development.