An automated Couette-type torsion-wire surface shear viscometer was used to measure the apparent interfacial shear viscosity of a pH 7 (I = 0.05 M) phosphate-buffered lysozyme solution in contact with n-tetradecane at a planar interface. When tristearin crystals were present in the oil phase and lysozyme in the aqueous phase, a higher initial apparent interfacial shear viscosity resulted than would be expected from the individual contributions of lysozyme and crystals alone. It is suggested that this synergism is due to the adsorption of hydrophobic groups of the lysozyme to tristearin crystals at the interface. Interactions between proteins and fat crystals are thought to be of significance in the stabilization of many food emulsions.