Radiolabeled stearic acid, (\beta)-casein, and lysozyme were used to construct (\mathrm{cpm}) versus surface radioactivity calibration curves to study the kinetics of adsorption of proteins at the air-water interface by the radiotracer method. It was observed that quantification of the rate and extent of adsorption of lysozyme was profoundly affected by the choice of the standard curve used. Whereas the calibration curve based on spreading (1{ }^{-14} \mathrm{C})-stearic acid at the air-water interface underestimated the surface concentration of lysozyme, that based on spreading of lysozyme resulted in overestimation. Even though (1-{ }^{14} \mathrm{C})-stearic acid and (\beta)-casein formed stable insoluble spread films at the air-water interface, the surface (\mathrm{cpm}) versus surface radioactivity calibration curves were different. These differences presumably arose from differences in the depth of the ({ }^{14} \mathrm{C}) nuclide from the air-water interface as well as from possible quenching of (\beta^{-})radiation by the protein matrix. Based on the observations it is proposed that the calibration curve obtained by spreading ({ }^{14} \mathrm{C})-labeled (\beta)-casein might be a better reference curve than the others to quantify the rate and extent of adsorption of proteins at the air-water interface using the radiotracer method. (9) 1993 Academic Press, Inc.