The metabolisable protein (MP) system (AFRC, 1992), adopted as the official method of describing the protein requirements of ruminants in the UK, introduces the concepts of effective rumen degradable protein (ERDP), digestible undegradable protein (DUP) and fermentable metabolisable energy (FME). For effective protein rationing, the aim is to match the supply of FME and ERDP, and to supply further protein requirements as DUP. This should lead to a better balance of energy and protein to meet the requirements of the animal. Diets are likely to have lower total protein contents than those presently available, leading to reductions in diet costs and nitrogen (N) excretion. The MP system was constructed primarily on a theoretical basis and this experiment was an initial step in validating it for beef cattle.