As previously reported, rabbit polymorphonuclear leucocytes contain an enzyme capable of hydrolyzing biosynthetic phenolphthalein mono-β-glucuronide. The effect of a number of inhibitors on cell-free preparations of this enzyme was studied. Many of the usual enzyme inhibitors were without effect, but certain dicarboxylic acids were strong inhibitors. These were (in order of activity): D-glucosaccharic acid, D-mucic acid, L-malic acid, mesotartaric acid, DL-malic acid, and L-tartaric acid. The nature of the inhibition was competitive. The Michaelis constant, Ks, was evaluated for phenolphthalein mono-β-glucuronide and values of Ki were estimated for glucosaccharic acid, mucic acid, DL-malic acid, and mesotartaric acid. Heparin in high concentration was slightly inhibitory, but the nature of the inhibition was noncompetitive. For each of the inhibitors studied, the extent of the inhibition was influenced by the hydrogen ion concen tration. At a pH greater than 4.5, the optimum for the enzyme under the conditions used, the degree of inhibition was less, and at a pH less than 4.5 the degree of inhibition was greater.