In plants, polyphenoloxidase undergoes an irreversible inactivation during the oxidation of o-diphenol to o-quinones. In the present paper, using latent polyphenoloxidase from iceberg lettuce (Lactuca sativa), in the presence of an activating agent, SDS, the kinetic parameters that characterize the enzyme during its action on the suicide substrate 4-tert-butylcatechol have been determined. The effect of pH has also been considered. It was seen that the presence of SDS in the reaction medium changed the kinetic parameters of the enzyme during suicide inactivation, this phenomenon depended on the SDS concentration up to saturating concentrations. Variations were also observed in the kinetic parameters at pH values below 5 where SDS provoked inactivation of the enzyme. This differing kinetic behaviour during suicide inactivation in the presence of SDS may be caused by the conformational changes provoked by SDS in the enzyme in latent state. Thus, polyphenoloxidases showing suicide inactivation would present an enzymatic activity resulting from the balance between the activation process and suicide inactivation.