Hydroxycitric acid was first isolated in the by-products of the beet sugar industry. To synthesize hydroxycitric acid, 1 mol. of transaconitic acid is neutralized and treated with 1 mol. HOCl (HgO + Cl2) with cooling. The calculated amt. of HC1 is added, Hg precipitated with H2S, the soln. concd. to 2.5 1., made alkaline with Ca(OH)2, and boiled l/2 hr. The mixture is allowed to stand for 1 wk., and crystals of the lac-tone of d, [iota]-allohydroxy-citric acid, m. 182[degree], are obtained. The acid mixture is neutralized with NaOH, the calculated amt. of CdCl2 added and the mixture autoclaved 1/2 hrs. at 130[degree]. From the Cd salt, the well crystallized lactone of d,[iota]-hydroxycitric acid, m. 152[degree], is obtained. To study the action of plant and animal dehydrogenases on these acids, citric acid-dehydrogenase was prepared from cucumber seeds, and iso-citric acid-dehydrogenase from liver and heart tissue. The reaction mixtures contain 1 ml. enzyme soln., 0.5 ml. methyl-ene blue soln. (1:5000) and 0.5 ml. M/100-M/1000 substrate. Allohydroxycitric acid is not attacked by the enzymes, while d, [iota]-hydroxycitric acid is dehydrogenated as rapidly as citric acid. , l-Hydroxycitric acid is split into its optically active components by dissolving the acid in 30 vols. water and adding powdered cinchonine until no more dissolves. The mixture is filtered and the salt of the dextrorotatory form crystallizes from the filtrate, while the levorotatory form remains in the mother liquor. The levorotatory component is not acted upon by the enzymes, while the dextrorotatory compo-nent is attacked as rapidly as is citric acid.