Allylsulfide caused an irreversible inactivation of ammonia monooxygenase (AMO) activity (ammonia-dependent O(2) uptake) in Nitrosomonas europaea. The hydroxylamine oxidoreductase activity (hydrazine-dependent O(2) uptake) of cells was unaffected by allylsulfide. Anaerobic conditions or the presence of allylthiourea, a reversible noncompetitive AMO inhibitor, protected AMO from inactivation by allylsulfide. Ammonia did not protect AMO from inactivation by allylsulfide but instead increased the rate of inactivation. The inactivation of AMO followed pseudo-first-order kinetics, but the observed rates did not saturate with increasing allylsulfide concentrations. The time course of recovery of AMO-dependent nitrite production after complete inactivation by allylsulfide required de novo protein synthesis. Incubation of cells with allylsulfide prevented the C label from C(2)H(2) (a suicide mechanism-based inactivator of AMO) from being incorporated into the 27-kDa polypeptide of AMO. Some compounds structurally related to allylsulfide were unable to inactivate AMO. We conclude that allylsulfide is a specific, mechanism-based inactivator of AMO in N. europaea.