The improvement of current anti-viral vaccines and the development of novel vaccines depends on increasing our understanding of viral attachment and fusion glycoproteins. Critical insight into understanding the antigenic structure of glycoproteins is provided by studying their interaction with monoclonal antibodies (mAbs). For a number of years we have studied the influenza virus hemagglutinin (HA) glycoprotein. This protein serves as a model for other proteins with similar functions (e.g., HIV gp160), and moreover, is important practically in its own right, as influenza still is a major cause of morbidity and mortality nationally, and internationally. Like many viral glycoproteins the HA is a homo-oligomer, consisting of three identical monomeric subunits. In the past year we continued to investigate the site of trimerization of newly synthesized HA. Our previously published findings suggested that HA trimerization occurs only after monomers are exported from the ER. In the past year we have used a number of novel inhibitors of ER to Golgi complex traffic to demonstrate immunocytochemically and biochemically that trimerization occurs in the ERGIC (acronymic for ER-Golgi Intermediate Compartment).