The effects of sodium dodecylsulfate (SDS) and Triton X-100 on the stability and activity of cutinase from Fusarium solani pisi were studied using the soluble substrate p-nitrophenylvalerate (PNPV). Incubation of the enzyme with SDS caused the formation of an inactive intermediate that could be reversibly activated by Triton X-100. The intermediate is long lived Out unstable, degrading more slowly to an irreversibly inactivated material. Degradation of native enzyme as well as the intermediate follows first-order kinetics. At concentrations below 1 mM, SDS carried an increase in K-m with little effect on V-max for PNPV resulting in reduction in the second-order rate constant. An increase in the second-order rate constant for phosphorylation by diethyl p-nitrophenylphosphate (E600) was observed with increasing SDS concentrations. These data suggest SDS causes local conformational changes in the active site that result in inhibition partial reversible unfolding, and subsequent inactivation.