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Vol. 49 GLUCURONIDE DECOMPOSITION IN THE RUMEN 215 2. The optimum pH for the decomposition of phenolphthalein glucuronide by the enzyme was 6*1.3. Irreversible inactivation of the enzyme occurred on incubation for a short period at a pH lower than 6 or higher than 7.4. On the assumption that the rate of liberation of phenolphthalein from the glucuronide was governed by a single reaction a value of 3 05 x 10m was obtained for the dissociation constant of the active enzyme-substrate complex. Excess substrate caused pronounced inhibition of the enzyme. The inactive complex contained two substrate molecules per active enzyme centre, and the second dissociation constant was 00126M. 5. Saccharate caused only slight inhibition of the enzyme in comparison with its effect on animal ,-glucuronidase.The authors wish to express their gratitude to Dr A. E Oxford for assistance and advice in microbiological aspects of this work, and to Dr A. T. Phillipson for the supply of sheep with rumen fistulae.