To understand the structure-function relationship of an anionic lipopolysaccharide emulsan, the effect of hydrodynamic volume on the emulsifying activity was investigated. As a result, it was found that the hydrodynamic volume of emulsan was an important factor in its emulsifying activity. The hydrodynamic volume was decreased by the addition of a positively charged polypeptide, and the emulsifying activity was decreased, but negatively charged or uncharged polypeptide had little effect. These results suggest that the conformation of the backbone of emulsan helps to govern its emulsifying activity.