The kinetics of competitive adsorption of beta-casein and lysozyme at the air-water interface has been studied using the radiotracer method. The presence of beta-casein in the bulk phase dramatically decreased the lag time for adsorption of lysozyme. However, the apparent diffusion coefficients of beta-casein and lysozyme in the binary mixture were exactly the same as those in single-component systems. The interfacial composition of the proteins at any given bulk concentration ratio was only affected by the relative rate of arrival of each protein at the interface and the molecular area available at the interface at the time of arrival. At bulk concentration ratios below 1:1, beta-casein and lysozyme formed a mixed monolayer; the ratio of beta-casein to lysozyme in the mixed monolayer increased as the ratio in the bulk phase was increased. However, at a beta-casein/lysozyme ratio of 2.8, the late arriving lysozyme was essentially excluded from the monolayer. It was also observed that when the average surface area per protein molecule of the mixed film reached a value of about 4500 angstrom2, the adsorption of lysozyme was abruptly inhibited, whereas the adsorption of beta-casein continued. Sequential adsorption experiments showed that bulk phase beta-casein could not displace adsorbed lysozyme, and lysozyme could not displace adsorbed beta-casein. Critical analysis of these results indicated that the extent of adsorption of these proteins from a bulk mixture is affected by the rate of arrival at the interface, and in this sense is not a thermodynamically-controlled competitive adsorption process.