The Mg salt of dipicrylamine, Mg(N(C6H2(NO2)3)2)2 is added to a soln. of acetylcholine Cl, and shining red, microscopic platelets of acetylcholine hexylate, C7H16O2N . C12H4O12N7, m. 183[degree], are obtained. Acetylcholine can thus be detected in a conc. of 1:10,000. Under the same conditions, choline forms needle shaped crystals of hexylate, C5H14ON . C12H4O11N7, which decomposes at 232[degree]-233[degree]. By the test, creatinine can be detected in a conc. of 1:20, the hexylate having the formula C4H7O . N3 . C12H5 . O12N7, and decomposing at 165[degree]-167[degree].