The behavior of the model proteins, lysozyme, myoglobin, and beta-casein, pretreated in urea and/or dithiothreitol, at air/solution interfaces was studied by surface pressure-area techniques. The data suggested that in the absence of pretreatments the globular proteins are only partially unfolded at the interfaces. The interfacial activity was enhanced by the pretreatment (lysozyme in 8 M urea with 0.2 M dithiothreitol and myoglobin in 8 M urea). The interfacial activity of casein, a random-coil type protein, was not influenced by the pretreatment (8 M urea), as it readily and completely unfolds at the interfaces. The unfolding of globular proteins at the interfaces is apparently restricted by both disulfide and noncovalent bonds. Pretreatment can relax those restrictions, resulting in more complete interfacial unfolding.