Bovine viral diarrhea (BVD) virus-specific antiserum eluted from an immunoabsorbent column was conjugated with horseradish peroxidase. The immunoperoxidase (IP) conjugate was used to stain BVD virus-inoculated and control tissue cultures processed for light and electron microscopies. Infected cells (in the inoculated cultures) viewed by light microscopy had diffuse staining of the cytoplasm of nonvacuolated cells and a more predominant staining associated with vacuoles as vacuolation occurred. The peroxidase label as demonstrated by electron microscopy was associated primarily with nonenveloped and membrane-associated viral particles. Viral particles 20 to greater than 100 nm (diam) were observed. Some of the particles which were greater than 30 mm in diameter contained multiple nuclear cores. The most intensely stained viral particles were outside of the cells.