Abstract A mixed spread monolayer composed of antigen and homologous antibody served as a model of lipid A—antilipid A IgG interaction at the air—water interface. Its behaviour was compared with that of lipid A—normal IgG. The immunoglobulins used were antilipid A IgG (s-IgG) and normal IgG (n-IgG). The study of the surface pressure—surface equilibrium area relationship of both IgG spread monolayers revealed that the s-IgG and n-IgG molecules occupied different areas, which may be attributable to different orientations on the surface. This was also confirmed by the different rheological dilatational properties of the two types of IgG. Equilibrium and dynamic measurements of mixed s-IgG/lipid A films indicated that their elastic and viscoelastic properties were dependent on the s-IgG/lipid A molar ratio; an immune complex being formed at molar ratio of x s-IgG = 0.1. Immune complex formation brought about a change in the surface rheological properties as shown previously.