Millon's test is given by any compound containing a phenolic hydroxy group. Consequently, any protein containing tyrosine will give a positive test of a pink to dark-red colour. The Millon reagent is a solution of mercuric and mercurous ions in nitric and nitrous acids ( CAUTION: MILLON'S REAGENT IS HIGHLY TOXIC AND HIGHLY CORROSIVE ). The red colour is probably due to a mercury salt of nitrated tyrosine.