A high-titer hiperimmune calf serum was obtained via the manifold hyperimmunization with local strains of the bovine pestivirus, cultivated in homologous cell cultures. Its globulin fractions were treated with fluorescein isothiocyanate to produce a specific conjugate. The specificity of the immunofluorescent serum was demonstrated through the comparative study of cell cultures infected with various viruses. The method worked out was employed to investigate tissue cross sections from diseased calves, whereupon the opportunity for the rapid and specific diagnosis of mucous disease was demonstrated. The replication dynamic of a bovine pestivirus strain in cell cultures was followed up.