A phospholipid monolayer was prepared at the air-water interface and different amounts of a lipid-tagged genetically fragmented antibody were incorporated into the lipid matrix. The molecular surface area of the film increased with antibody concentration and reached a maximum at an antibody mole fraction of 0.03. Atomic force microscopy showed that the antibody fragment was segregated into protein-rich domains with a mean diameter of about 12 nm. In-situ measurements with surface plasmon resonance displayed a specific binding of antigen-a binding that was not observed to the pure lipid film.