Immunoperoxidase and immunofluorescence techniques detected and localized infectious bursal disease virus (IBDV) in formalin-fixed paraffin-embedded sections of the bursa of Fabricius of experimentally and naturally infected chickens. The primary antibody was a monoclonal antibody that bound all IBDV serologic subtypes, including the GLS isolate. Both techniques were valuable in detecting IBDV. The presence and severity of microscopic lesions in the bursa correlated with the location and number of positive IBDV-infected cells as measured by either test. Mild vaccine strains induced minimal microscopic lesions and resulted in a few cells positive by either test. In contrast, virulent IBDV produced widespread lymphoid necrosis and numerous cells positive by both assays. The immunoperoxidase test was more useful than the immunofluorescence test, since the same section could be stained and examined by immunoperoxidase and then restained and examined for microscopic pathology. The presence of immunoperoxidase-positive stained cells associated with areas of microscopic pathology suggested that microscopic lesions were induced by IBDV.