Monoclonal antibodies to glycoprotein and protein antigens of infectious laryngotracheitis virus (ILTV) were divided into five groups on the basis of their reactivity in immunofluorescence and Western blotting. Group I antibodies recognised a single band of 60 K and Group II antibodies recognised bands of 205, 160, 115, 90 and 85 k in Western blotting. In immunofluorescence both these groups of antibodies reacted with antigens located in the cytoplasm of fixed virus-infected cells and they also reacted with unfixed cells, suggesting that these antigens are on the surface of virus-infected cells. While Group I monoclonal antibodies did not react with extracts of tunicamycin-treated cells, some Group II antibodies recognised bands of decreased molecular weight compared to those present in untreated cells. The reactivity of the Group II antibodies with extracts of tunicamycin-treated cells suggested that they recognised at least three different epitopes which was confirmed by ELISA additivity assays. Monoclonal antibodies of Group III, Group IV and Group V recognised several low molecular weight proteins from 45 to 24 k. Immunofluorescence studies showed that these were nuclear and cytoplasmic antigens that were not present on the surface of virus-infected cells.