A quick direct fluorescent antibody test incorporating a monoclonal antibody, 1NDW14D, reacting with the nucleoprotein of infectious hematopoietic necrosis virus (IHNV), revealed the presence of the 1989 Dworshak isolates of the virus 8 h after Chinook salmon embryo (CHSE-214) cells were incubated with fish homogenates containing the virus. An indirect fluorescence assay was performed to compare the ability of the anti-nucleoprotein antibody, 1NDW14D, with that of an anti-glycoprotein antibody, 3GH135L, to detect IHNV. Although the nucleoprotein of IHNV is synthesized before the glycoprotein, both antibodies detected the presence of virusinfected cells with equal efficiency when tested on several isolates of IHNV. Both antibodies stained caps of glycoprotein or nucleoprotein in the cytoplasm contiguous with the plasma membrane of infected cells within 6-8 h after infection.