This paper describes a method for the specific quantification of whole virions of foot-and-mouth disease (146S) in the presence of virus subunits (12S). The method involves the use of virus neutralising monoclonal antibodies directed against a linear epitope of the VP1 loop region of a type O virus. The monoclonal antibodies were used as both capture and detecting reagents (labelled with horse radish peroxidase) in a sandwich ELISA. Such monoclonal antibodies also have the advantage that they do not detect viruses containing proteolytically cleaved VP1, thus the assay system is ideal for estimation of whole particles in vaccine manufacture where the immunogenicity of the vaccine depends on virus integrity (whole virions being present) and uncleaved capsid protein. VP1. Other combinations of different anti-type O FMD virus monoclonal antibodies used as capture and detecting reagents were also examined. The system could be adapted to on-line continuous testing of virus being produced during a manufacturing run allowing maximisation of virus yield and quality control.