The benzoylarginine peptidase of Treponema denticola (strain ASLM; a human oral spirochaete) was progressively and irreversibly inactivated by 1-(ethoxycarbonyl)-2-ethoxy-1, 2-dihydroquinoline, a carboxyl-group reagent. At acidic pH values, reaction of one mole of the modifier per active site of the enzyme resulted in total inactivation of the enzyme. Assuming that this modifier is a specific carboxyl reagent, the data suggest that the inactivation of the T. denticola benzoylarginine peptidase was caused by the modification of one carboxyl group located close to the active site of the enzyme. Results obtained with Woodward's reagent K ( N -ethyl-5-phenylisoxazolium 3’-sulphonate) supported these findings. Carbethoxylation with diethylpyrocarbonate effectively inactivated the enzyme, and addition of hydroxylamine at pH 7.0 restored the activity almost totally, suggesting that the pyrocarbonate had reacted with tyrosyl or histidyl residues.