A new methodology has been developed for studying the adsorption behavior of proteins at oil-water interfaces. This technique employs the radiotracer method for monitoring adsorption of C-14-labeled proteins at the oil-water interface. The uniqueness of the new method lies in the formation of a 1000 Angstrom thick triglyceride oil film on the water surface. beta-casein was used to generate a standard curve for relating interfacial radioactivity (mu Ci/m(2)) to cpm at the oil-water interface. Adsorption isotherm of beta-casein was determined in the bulk protein concentration range 1.5 x 10(-5)-3.8 x 10(-3)% by weight of solution. The saturated monolayer coverage was found to be about 7.3 mg/m(2). This value was quite different from other values reported in the literature. Adsorption studies with another protein, lysozyme, at the oil-water interface also revealed a high surface concentration of 3.0 mg/m(2). The most significant difference between the adsorption of beta-casein at the oil-water and air-water interfaces was the lack of an induction period for the development of interfacial pressure in the former. This difference may be attributed to the attractive dispersion interaction between protein molecules and the oil phase.