This report describes the preparation of six monoclonal antibodies (MAbs) raised against a British isolate of porcine reproductive and respiratory syndrome virus (PRRSV), their characterization in terms of protein specificity and their reactivity with different PRRS viruses from Europe and the USA. Radioimmunoprecipitation and Western blotting studies of MAb reactivity with proteins from cell lysates of infected cells and purified virus revealed that four of the six MAbs (WBE1 and WBE4-6) precipitated a 15 kDa viral protein. Further studies using in vitro translated products of the Lelystad virus genome showed that this protein was the product of ORF7, the putative nucleocapsid protein. The specificity of another MAb, WBE2, was found to be for a 45 kDa protein, determined to be the product of ORF3 and demonstrated to be present in purified virion preparations. The protein specificity of the sixth MAb, WBE3 could not be determined. Thirty-three PRRSV isolates from Europe and the USA were grown in alveolar macrophages and examined by immunoperoxidase staining, using the panel of six MAbs. All European isolates were recognized by the four MAbs specific for the putative nucleocapsid, but the viruses showed different patterns of reactivity with WBE2 and WBE3. Furthermore, these two MAbs stained only a small proportion of the cells infected with certain isolates, suggesting that a single isolate may be antigenically heterogeneous. No MAbs bound to US isolates, indicating a consistent antigenic difference between the putative nucleocapsid of US and European isolates. Detergent extraction of cell lysate antigen abrogated the binding of WBE1-3, suggesting that the epitopes are conformation dependent.