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 residues which occur in nearly all proteins.
Millon's test is not specific for proteins (it actually detects phenolic compounds), and so must be confirmed by other tests for proteins such as the biuret test and the ninhydrin reaction.
The reagent is made by dissolving metallic mercury in nitric acid and diluting with water.
The test was developed by the French chemist Auguste Millon (1812–67).