The improvement of current antiviral 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. This conclusion was based largely on localization studies performed with a standard immunofluorescence microscope. In the past year, we have increased the resolution of detection by using a laser confocal scanning microscope.