In the present study we used the film balance technique to study the interaction between beta-lactoglobulin and distearoylphosphatidic acid, distearoylphosphatidylcholine, and dipalmitoylphosphatidic acid at the air/water interface. The pH and the salt concentration of the subphase were chosen as variables to determine the nature of the interaction. The area increase upon adsorption at constant surface pressure was measured as a function of the surface pressure and analyzed by applying a simple first-order kinetic model. The highest rate of adsorption was found for adsorption into distearoylphosphatidic acid monolayers. The results show that not only electrostatic but also hydrophobic interactions are important for the incorporation of beta-lactoglobulin into phospholipid monolayers. The results were found to agree qualitatively with our previous study on the interaction of beta-lactoglobulin with the same phospholipids dispersed in aqueous solution.