Using diets based on rat amino acid requirements, containing 2.0% nitrogen and 4.1 digestible kcal/g, the dietary levels of ten essential amino acids required for normal post-weaning growth of Swiss X Carworth Farms No. 1 crossbred mice were estimated. The response criteria used were growth rate, dry matter intake, energy and protein contents of body weight gain, protein efficiency ratio and apparent biological value. Maximum responses were obtained when the dietary dry matter contained less than or equal 0.3% arginine, 0.2% histidine, 0.4% isoleucine, 0.7% leucine, less than or equal 0.4% lysine, 0.5% methionine, less than or equal 0.4% phenylalanine, 0.4% threonine, less than or equal 0.1% tryptophan and 0.5% valine.