Various combinations of polyclonal and neutralizing monoclonal antibodies (MAbs) of distinct specificity were evaluated as capture or detecting antibodies in an orthopox virus antigen ELISA. Acceptable results were achieved in an assay based on polyclonal antibodies. A 10 times higher sensitivity, however, was obtained using a combination of one monoclonal catching antibody reactive with viral envelope epitopes and polyclonal detection antibodies. This configuration proved to be superior in sensitivity to all others. Specificity was confirmed with 5 vaccinia virus strains (including one recombinant virus) and 8 species of orthopox viruses. Monkeypox and mousepox virus reacted exclusively in the polyclonal assay due to a lack of the specific epitope for the monoclonal antibody. The detection limit compared to the infectivity titer amounted to 10(3)-10(4) in the monoclonal/polyclonal and to 10(4)-10(5) TCID50/0.1 ml in the polyclonal combination. The correlation between infectivity- and ELISA-titer was demonstrated by a study of the replication cycle of the rabbitpox virus Uetrecht in the permanent rabbit kidney cell line RK-13. With the established ELISAs vaccinia virus could also be detected in organ suspensions of lethally infected NMRI-mice, depending on the level of infectivity.