A study was performed with a surface force apparatus (SFA) in which the automation of a mica surface displacement at very low speed and the use of multiple beam interferometry yield force profiles and variations of the refractive index of the macromolecular layers adsorbed between the two mica plates of the device. The performances of such an SFA are illustrated by the identification of the two molecular types of the plant lectin concanavalin A (Con A)when the anisotropic dimer Con A is converted into an isotropic tetramer form. Thus, the SFA can be a good model device for studying macromolecular interactions submitted to a variable compression stress in a confined space.