Thirteen hybridoma cell lines which secrete monoclonal antibodies (MCAs) against swine fever virus (SFV) strain Brescia were produced. The hybrid cells resulted from fusion of P3X63-Ag8.653 myeloma cells with splenocytes of Balb/c mice which had been immunized with purified SFV. Screening of supernatant fluids was performed by an indirect enzyme-linked immunosorbent assay (ELISA) and an immunoperoxidase monolayer assay (IPMA). The IPMA, and an immunoperoxidase test (IPT) performed on cryostat sections, were used to characterize these MCAs on several pestivirus strains. All MCAs reacted to a varying degree with all but one of the SFV strains tested. None of the MCAs reacted with the bovine viral diarrhoea virus (BVDV) strains. Two MCAs are now used routinely in the differential diagnosis between infections with field strains of SFV, and the Chinese vaccine strain and BVDV strains.