Millon's reagent is an analytical reagent used to detect the presence of tyrosine, the only amino acid containing a phenol group.In Millon’s test, the phenol group of tyrosine is first nitrated by nitric acid in the test solution. Then the nitrated tyrosine complexes mercury (I) and mercury (II) ions in the solution to form a pink to brick red precipitate or a solution. Some proteins containing tyrosine initially forms a white precipitate that turns red when heated, while others form a red solution immediately. Both results are considered positive.