An amino acid mixture comprising a balanced mixture of natural essential amino acids, free from dicarboxylic acids and substantially free from inactive isomeric amino acids, is produced by extracting an aqueous protein hydrolysate with a 3 to 5 carbon alcohol, recovering from the alcohol extract an amino acid fraction consisting of substantially pure natural essential monoamino-monocarboxylic acids, combining said fraction with substantially pure natural essential basic amino acids, tryptophane, crystalline essential amino acids present but difficient in said fraction, and with glycine in an aqueous medium.  According to the example, casein is hydrolysed with 20 per cent sulphuric acid, Ba(OH)2 added to apH 5.4 and the precipitated BaSO4 removed.  The solution is cooled to 5 DEG  to 10 DEG  C. for 16 hours to precipitate tyrosine, which is purified over charcoal.  The residual solution is extracted with n-butanol, the butanol extract concentrated in vacuo to remove water and precipitate the monoamino-monocarboxylic acids (fraction A-leucine, isoleucine, phenylalanine, valine, methione and threonine).  The spent liquor remaining after the butanol extraction is concentrated to remove dissolved butanol, rediluted with water and percolated through an absorbent column, e.g. Amberlite IR100, on which the basic amino acids (fraction B-arginine, histidine and lysine) are retained, washed free from dicarboxylic acids, eluted therefrom with sulphuric acid and recovered by the addition of Ba(OH)2 and subsequent concentration in vacuo.  The fractions (A) and (B) are assayed for individual amino acid content and admixed in proportions such that together with added tryptophane, other essential crystalline amino acids present but deficient in said fractions and glycine, a mixture of desired composition is obtained, i.e. the total solids may comprise over 70 per cent by weight of natural essential amino acids. The mixture so formed is dissolved in water, purified over charcoal to remove pyrogens and sterilized by autoclaving under nitrogen.  Any 3 to 5 carbon atom alcohol may be used in the extraction of the monocarboxylic acids and absorbents such as "Zeokarb" (R.T.M.) and "Filtrol Neutral" (R.T.M.), may be employed for separating the basic amino acids.  Proteins other than casein, which have a high amino acid content, e.g. lactalbumin, fibrin, blood plasma protein and yeast protein, are also suitable and may be hydrolysed by acids other than sulphuric, e.g. hydrohalic, phosphoric and sulphonic acids.ALSO:An amino acid mixture for parenteral, oral and rectal administration comprising a balanced mixture of natural essential amino acids, free from dicarboxcylic acids and substantially free from inactive isomeric amino acids, is produced by extracting an aqueous protein acid hydrophate with a 3 to 5 carbon alcohol, recovering from the alcohol extract an amino acid fraction consisting of substantially pure natural essential monoamino monocarbooxylic acids, combining said fraction with substantially pure natural essential basic amino acids, tryptophane, crystalline essential amino acids present but deficient in said fraction and with glycine in an aqueous medium.  According to the example, casein is hydrolyzed with 20 per cent sulphuric acid, barium hydroxide added to pH5.4 and the precipitated BaSo4 removed.  The solution is cooled to 5 to 10 DEG C. for 16 hours to precipitate tyrosine which is purified over charcoal.  The residual solution is extracted with p -butanol, the butanol extract concentrated in vacuo to remove water and precipitate the mono-amino monocarboxylic acids (fraction A-leucine, isoleucine, phenylalamine, valine, methione and threonine).  The spent liquor remaining after the butanol extraction is concentrated to remove dissolved butanol, rediluted with water and percolated through an absorbent column (e.g. Amberlite 1R100) on which the basic amino acids (fraction B-arginine, histidine and lysine) are retained, washed free of dicarboxylic acids, eluted therefrom with sulphuric acid and recovered by addition of barium hydroxide and concentration in vacuo.  The fractions (A) and (B) are assayed for individual amino acid content and admixed in proportions such that together with added tryptophane, other essential crystalline amino acids present but dificient in said fractions and glycine, a mixture of desired composition is obtained, i.e. the total solids containing over 70 per cent by weight of natural essential amino acids.  The mixture so formed is dissolved in water, purified over charcoal to remove pyrogens and sterilized by autoclaving under nitrogen.  Any 3 to 5 carbon atom alcohol may be used in the extraction of the mono-amino monocarboxylic acids and absorbents such as "Zeo Karb H" (R.T.M.) and "Filtrolneutral" (R.T.M.) may be employed for separating the basic amino acids.  Proteins high in amino acid content other than casein, e.g. lactabumin, fibrin blood plasma protein and yeast protein are also suitable and may be hydrolysed by acids other than sulphuric, e.g. hydrohalic, phosphoric and sulphonic acids.