Antigenic and genetic analyses were performed in order to establish relationships between the noncytopathogenic (ncp) and the cytopathogenic (cp) bovine viral diarrhoea viruses (BVDV) involved in the induction of a case of experimentally induced "late-onset" mucosal disease (MD) symptoms. The persistent ncpBVDV, the cpBVDV used for superinfection (strain TGAC) and the virus isolates from faeces (cpX) were examined using an immunoplaque test (IPT) to distinguish between cp and ncp virus populations. The cp populations were cloned by plaque purification and found to be free of ncpBVDV when using the IPT. The cpBVDV clones and the persistent ncpBVDV were analysed in an enzyme immunoassay on heat-fixed infected cells (IM-EIA) and in a neutralization test using a panel of 27 monoclonal antibodies against the E0 (gp48) and E2 (gp53) viral glycoproteins. It was found that strain TGAC contained two antigenically distinct subpopulations of cpBVDV (TGAC-B1 and TGAC-B2). The endogenous ncpBVDV and the cpX clones had the same reactivity pattern in both tests. In addition, p80 gene duplications in the genomes of the cpBVDV clones were analysed using the polymerase chain reaction and subsequent restriction enzyme analysis of the amplicons. The clones analysed from TGAC-B1 and those from cpX had gene duplications of identical sizes showing the same restriction enzyme patterns. Our results suggest that the cpBVDV which finally lead to "late-onset" MD arose by recombination and/or by mutations of the cpBVDV used for superinfection.