Molecular exchange kinetics between a monolayer of antibody molecules formed on the air-water interface and the protein solution was studied by means of fluorescent labeling. It was shown that there is no inclusion of dissolved molecules in the previously formed monolayer during even 6 h of exposure regardless of monolayer surface density. The surface activity of IgG and horseradish peroxidase molecules was studied by means of surface compression isotherms, and the specific biological activity of the monolayers formed from these proteins was measured by enzyme and immunoassay techniques. It was shown that the surface activity of the proteins increases while specific biological activity decreases with exposure of the molecules on the water surface. Since the same effects were caused by denaturing agents, we propose that the surface activity of the proteins and the absence of surface-volume exchange are due to partial unfolding of the molecules which takes place on the water surface. Two models of the partial unfolding are discussed : complete denaturation of some part of the molecules and partial unfolding of each molecule. The process of surface denaturation was shown to be slow and controllable. One can achieve a pronounced increase of protein surface activity with low degradation of the specific biological activity of the monolayer; thus, it can be used in the practice of protein Langmuir film deposition.