Proteins are widely used for the physical stabilization of emulsions and foams. Stabilization strongly depends on the interfacial properties of the proteins. In many cases likely a kind of interfacial gel layer is formed. This paper deals with fracture properties of such interfacial gelled protein layers as studied by the overflowing cylinder technique. In the first part of the paper, equations are presented for the calculation of the fracture stress of these protein layers from experimental data. This is illustrated in the second part of this paper for ovalbumin, glycinin, beta-casein, and beta-lactoglobulin. Obtained fracture stresses are on the order of 2 x 10(5) Pa and dependent on the type of protein. Protein properties such as conformation seem to be important aspects, determining the height of the fracture stress. Although interfacial protein gels are fracturable, relaxation phenomena within the gelled protein layers are also important.