Increases in the viscosity of solutions of different proteins varying over a wide range of molecular shape and size have been studied in a cone and plate viscometer. The protein concentrations used were lower than those needed to produce viscosities of the solutions significantly greater than the solvent medium. The high values of shear stress finally reached are attributed to a shear-induced formation of an intermolecular structure resulting from protein-protein interactions at the air water interface. The rate of formation of this structure was found to increase with ionic strength, indicating that the rate of adsorption at the interface may influence the development of high shear stress values. However, this effect also shows that hydrophobic intermolecular interactions and the conformation of the protein molecules in solution are important factors in the development of the structure. The presence of small amounts of detergents was found to be able to prevent increases in shear stress. The fact that the double-chained lipid lecithin does not produce this effect, even in high concentrations, may be due to its ability to take part in protein-lipid-protein interactions.