Many ovine pestiviruses from Britain and a number of atypical porcine isolates are largely unrecognised by monoclonal antibodies (mAbs) specific for reference strains of classical swine fever virus and bovine viral diarrhoea virus (BVDV). Additional mAbs have therefore been produced using some of these "unreactive" pestiviruses. Two of the viruses used were atypical porcine isolates (strains 87/6 and Vosges), whilst another had been isolated from a sheep (59386). Thirty-three mAbs were selected, none of which recognised two reference strains of BVDV, but three of which recognised the Alfort strain of classical swine fever. On the basis of radioimmunoprecipitation they were considered to be directed at one of three different pestivirus proteins (gp 53, gp 48 or p 125). Three virus subgroups were evident when the mAbs were used to type 16 ovine and two atypical porcine pestiviruses. One subgroup contained the Vosges and 59386 viruses and four ovine field isolates. The second subgroup comprised the 87/6 virus, the Moredun and Aveyron reference strains of border disease virus and four further ovine field isolates. Three of four ovine viruses making up the third subgroup had been previously categorised as BVDV-like and were largely unrecognised by the new mAbs. The findings were in agreement with previous attempts to segregate some of the same viruses using partial genomic comparisons or cross-neutralization tests.