The rate of oxidation of β-lactoglobulin with ferricyanide decreases markedly as the concentration of sodium dodecyl sulfate is increased from 0.25 to 1.0%, whereas the detergent concentration has little effect on the rate of oxidation of this protein by o-iodosobenzoate and 2,6-dichlorophenol indophenol. It is suggested that the structure of this protein in sodium dodecyl sulfate, coupled with the steric effect of bound detergent molecules, hinders the formation of a transition state involving the sulfhydryl group, a heavy metal ion, and the ferricyanide ion.