Micelles of sodium dodecyl sulfate (SDS) are significantly retarded by the addition of a small amount of dodecyl alcohol to a sample solution in SDS-polyacrylamide gel electrophoresis. The phenomenon can be ascribed to the decrease in charge density due to the incorporation of dodecyl alcohol into SDS micelles. The effect is extended to SDS-protein polypeptide complexes when the amount of SDS micelles is insufficient to accomodate the dodecyl alcohol. A similar effect is likely to occur when SDS-polyacrylamide gel electrophoresis is applied to a sample containing lipophilic materials.