The glycoproteins E-rns of classical swine fever virus (CSFV) and E-rns and E2 of bovine viral diarrhoea virus (BVDV) are shown to be located at the surface of infected cells by the use of indirect immunofluorescence and by cytofluorometric analysis. The positive immunostaining of the cell surface was further analysed by immunogold electron microscopy and it could be shown that only extracellular virions were labelled. Gold granules were not seen at the cellular plasma membrane. In contrast to BVDV E2, the CSFV E2 of virions sticking to the plasma membrane was not accessible to the respective monoclonal antibodies. However, CSFV particles isolated from culture supernatant were able to bind both monoclonal anti-E-rns and anti-E2 antibodies. For CSFV and BVDV, binding of anti-E-rns antibodies to the virions was more pronounced than that of anti-E2. This finding was unexpected since E2 is considered to be the immunodominant glycoprotein.