Reducing the protein content of commercial diets in Europe would reduce environmental nitrogen pollution. It has been demonstrated that this could be achieved by employing crystalline amino acid supplementation (e.g. Chavel and Granier, 1994; Fuller et al, 1979a, b). Accurate information on the true ileal digestible amino acid requirements of the modern lean genotype pig is necessary in order to ensure optimal production at low protein intakes. The objective of this study was to identify the optimum ileal digestible methionine:lysine (met:lys) ratio to achieve maximal growth and food conversion efficiency of the lean genotype pig given low protein diets.