Novel equipment, the Cambridge Interfacial Tensiometer (CIT), was designed and used to directly determine the mechanical properties of a beta-lactoglobulin film adsorbed at the air-water interface. The CIT enables a true stress-strain curve to be determined for interfacially adsorbed protein films, through a two-dimensional analogy of conventional 'Instron' testing. The surface elasticity modulus for beta-lactoglobulin films adsorbed at the air-water interface, and aged for a period of 1 h, was found to be of order 100 mN/m. Films showed considerable 'self-healing' ability at large strains due to protein transport into the interfacial region, particularly at the highest protein concentration studied (1.0 mg/ml). Surface elasticity modulus increased with protein concentration, although a 31% enhancement in the maximum stress transmitted through the protein film resulted when sub-interfacial beta-lactoglobulin concentration was reduced from 1.0 to 0.01 mg/ml. Reduced competition for interfacial space at lower protein concentrations is believed to result in greater conformational change and hence entanglement on adsorption to the clean interface. Disruption of the hydrophobic interactions of protein molecules using non-ionic surfactant (Tween 20) resulted in a complete loss of lateral force transmission through the layer of adsorbed material. This is the first study to establish the mechanical properties of an adsorbed protein film using conventional stress-strain approaches to high material deformations. It shows that resistance of a film to small deformations is not, necessarily, correlated with the maximum stress that can be transmitted through the film. The CIT instrument provides a generic alternative to existing surface rheology tests, which are limited by the need for assumptions during data interpretation and by inhomogeneities in the local shear rate for viscoelastic test materials such as protein films.