Millon's reagent is an analytical reagent used to detect the presence of soluble proteins. A few drops of the reagent are added to the test solution, which is then heated gently. A reddish-brown coloration or precipitate indicates the presence of tyrosine

Tyrosine

Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

 residue which occur in nearly all proteins.

Millon's test is not specific for proteins (it actually detects phenolic

Phenols

In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of a hydroxyl group bonded directly to an aromatic hydrocarbon group...

 compounds), and so must be confirmed by other tests for proteins such as the biuret test

Biuret test

The biuret test is a chemical test used for detecting the presence of peptide bonds. In the presence of peptides, a copper ion forms a violet-colored complex in an alkaline solution...

 and the ninhydrin

Ninhydrin

Ninhydrin is a chemical used to detect ammonia or primary and secondary amines. When reacting with these free amines, a deep blue or purple color known as Ruhemann's purple is produced...

 reaction.

The reagent is made by dissolving metallic mercury

Mercury (element)

Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

 in nitric acid

Nitric acid

Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...

 and diluting with water.

The test was developed by the French chemist Auguste Millon (1812–67).

See also: Benedict's solution