1. The enzymic activity of glucose oxidase was determined as a function of pH and sodium n-dodecyl sulphate (SDS) concentration. 2. Glucose oxidase is not deactivated by SDS at pH 6 even after prolonged incubation, but is deactivated at pH 4.3 and 3.65. 3. Sedimentation-rate analysis showed that glucose oxidase dissociates into its two subunits at pH 5 and below, and sedimentation-equilibrium experiments in the presence of SDS gave a subunit molecular weight of 73,500. 4. SDS binds to glucose oxidase in acid solutions; specific binding occurs ap pH 3.65, but at pH 6 only co-operative binding was observed. 5. Glucose oxidases in which some of the carboxy groups were blocked with glycine methyl ester were deactivated by SDS at pH 6.0; the rate of deactivation increased with the extent of esterification. 6. Deactivation of esterified glucose oxidases correlated with thermal analysis of the initial SDS interaction, the exothermicity of the interaction increasing with the extent of esterification. 7. The results show that carboxy groups confer resistance to deactivation by SDS on glucose oxidase by screening cationic residues and inhibiting specific interactions that facilitate dissociation into subunits.