Experimental studies of dynamic and equilibrium surface tensions of aqueous beta-casein solutions were performed by using the pendent drop and bubble techniques. A comparison of the results shows good agreement at higher protein concentrations. However, at low protein concentrations a significant difference between the data is observed, caused by the loss of protein molecules from the bulk at the drop surface due to the limited solution reservoir. In bubble experiments the reservoir is larger by more than 2 orders of magnitude so that losses become essential at much lower bulk concentrations. Loss of protein and the diffusional transport in the limited reservoir of a small drop lead to the formation of an adsorption layer with properties quite different from those obtained at a bubble surface.