A Couette-type torsion wire surface shear viscometer was used to measure the apparent interfacial shear viscosity of pH 7 (I = 0.05 M) buffered solutions of lysozyme, sodium caseinate, and Tween-40 in contact with either n-tetradecane or purified sunflower oil at a planar interface. When proteins were present in the aqueous phase and tristearin crystals in the oil phase, there was a synergistic increase in the interfacial shear viscosity over the sum of each component in the absence of the other. The magnitude of the increase appeared to be independent of the type of protein but dependent on the nature of the oil phase. This increase in the interfacial shear viscosity was found not to be due simply to the protein reducing the interfacial tension and thus affecting the adsorption behavior of the fat crystals. When the aqueous phase contained a small-molecule surfactant (Tween-40) instead of protein, but at the same interfacial tension as the sodium caseinate system, a significantly smaller increase was observed in the interfacial shear viscosity than in the protein system. It therefore seems likely that when proteins are present, hydrophobic peptide residues interact with the tristearin crystals at the interface.