The effects of temperature on the energy of activation, the enthalpy of binding, and the apparent equilibrium constant for the overall reaction catalyzed by L-histidine ammonia-lyase (EC 4.3.1.3) have been determined. A temperature-dependent transition (at approximately 30°) involving multiple forms of the enzyme is also discussed.The enthalpy of binding is exothermic above 30° and endothermic below this transition temperature. In addition, a bend is observed in the Arrhenius plots at approximately 34°. The combination of these effects suggests the existence of at least two forms of the enzyme.