The viral glycoproteins produced in cells infected with either vaccine strain or virulent isolates of infectious laryngotracheitis virus, an avian herpesvirus, were identified by in vitro labeling using [14C]glucosamine and [14C]mannose. Chicken antisera to the vaccine strain and to a virulent isolate, and rabbit antisera to the vaccine strain, immunoprecipitated four major viral glycoproteins of 205, 115, 90, and 60K mol wt. Additional glycoprotein bands were recognized by immune chicken and rabbit sera in Western blotting using a glycoprotein fraction purified from extracts of virus-infected cells. Monoclonal antibodies to the immunogenic glycoproteins were produced and characterized by immunoprecipitation and Western blotting. One group of monoclonal antibodies reacted only with the 60K glycoprotein, by both techniques, while a second group reacted with the 205, 115, and 90K glycoproteins in immunoprecipitation and with additional bands of 85 and 160K in Western blotting.