Two-step solution competition assays were performed in solution with influenza type A virions and hemagglutinin (HA)specific neutralizing monoclonal antibodies (mabs), These demonstrated that the binding of one molecule of IgG mab per HA trimer prevented the binding of mabs directed against other antigenic sites on the HA (site A, site B, or site D), even though these are topographically separate and antigenically independent. Furthermore the same procedures showed that one molecule of mab per trimer prevented the binding of polyclonal HA-specific IgG obtained from the serum of rabbits immunized with whole virus. This restricted binding is clearly a property of the intact virion, since others using purified HA have shown that up to four IgG molecules of different specificities can bind per trimer. Since the surface area of the globular head of the trimer is equivalent to approximately 10 nonoverlapping antibody footprints, it is not understood how one prebound IgG molecule prevents the binding of other IgG molecules. (C) 1994 Academic Press, Inc.