The adsorption of antibodies against C-reactive protein onto a monolayer of octadecylamine has been studied at the air/water interface by following the change in area of the monolayer at different surface pressures. Binding of antibodies was dependent both on the packing of the film and on the antibody concentration in the aqueous subphase. An equilibrium was reached above an antibody concentration of 2 mg/dm(3). The monolayer was stabilized by the antibodies. The mixed monolayer could be transferred onto a quartz crystal microbalance. However, the high nonspecific binding of bovine serum albumin makes it difficult to determine the interaction of the antigen with the octadecylamine/antibody layer.