Sixteen monoclonal antibodies (Mabs) which immunoprecipitated the haemagglutinin neuraminidase (HN) of chorio-allantoic membrane-grown avian paramyxovirus-3 (PMV-3) of British turkeys were produced. Thirteen were PMV-3 specific. Three were nonspecific because they also bound to other viral proteins and to bovine kidney cells treated with neuraminidase enzyme or infected with influenza virus. The thirteen specific Mab defined four antigenic regions A-D by competition and variant selection assays. Region A was subdivided into five epitopes and region B into two epitopes. The thirteen Mab neutralized and were active in haemagglutination inhibition (HI) and twelve were active in haemolysis inhibition (HLI) tests. Neuraminidase inhibition (NI) was epitope-dependent. Mabs to five of the epitopes A1, A2, A3, A5, and C bound to the 1981 British turkey isolates but not the 1968 American turkey isolate. The Mab to epitope A4 bound to both viruses. The Mabs to epitopes B1, B2, and D also bound to a parakeet isolate of PMV-3 which was the third PMV-3 tested. The Mab to B2 gave identical titres to all three viruses and had HI, HLI, and NI activities. This made it a potential diagnostic reagent for avian PMV-3 viruses. One of the nonspecific Mabs bound to lactose-like moieties as reported on influenza virus and one to maltose-like moieties as on retroviruses. Immunoglobulin from all three nonspecific Mab had some HI activity.