Monoclonal antibodies were produced against two distinct Thai dengue-2 (DEN-2) virus strains isolated in 1980 from dengue haemorrhagic fever patients. Nine of 36 hybridomas produced monoclonal IgG antibodies which reacted in radioimmune precipitation assays with the NS1 non-structural protein (42,000 mol. wt.) from DEN-2-infected C6/36 (Aedes albopictus) cells. The virus specificity of NS1-reactive monoclonal antibodies was determined by indirect immunofluorescence assays using LLC-MK2 cells infected with either the Thai 1980 DEN-2 isolates, prototype DEN viruses (four serotypes), Japanese encephalitis (JE), Murray Valley encephalitis, West Nile, Wesselbron or Tembusu viruses. Eight of the monoclonal antibody preparations were DEN-2-serotype specific. One preparation defined a special serological relationship between DEN-2 and JE viruses. Four preparations had detectable complement fixation titres using Thai DEN-2 virus antigen. Six spatially unique epitopes were identified using competitive binding assays.