Viruses from six antigenic groups of arthropod-borne viruses [Crimean--Congo haemorrhagic fever (CCHF), Nairobi sheep disease (NSD), Qalyub (QYB), Sakhalin (SAK), Dera Ghazi Khan (DGK) and Hughes (HUG) serogroups], some previously categorized as bunyavirus-like viruses and others previously ungrouped, have recently been assembled by serological analyses into a new genus of viruses (Nairovirus genus) in the Bunyaviridae. Molecular studies of the virion RNA and viral polypeptides have been undertaken with representative members of the different serogroups [Hazara (HAZ) and Congo (CON) viruses, CCHF group; Dugbe (DUG) virus, NSD group; QYB, Omo and Bandia (BDA) viruses, QYB group; Avalon (AVA) virus, SAK group; DGK and Abu Mina (AM) viruses, DGK group; and HUG virus, HUG group]. In agreement with a recent study of QYB virus and in part agreement with an earlier report on DUG virus, the results of these molecular analyses indicate that nairoviruses have: (i) three virion RNA species (large, L, medium, M, and small, S) with apparent mol. wt. of 4.1 x 10(6) to 4.9 x 10(6), 1.5 x 10(6) to 1.9 x 10(6) and 0.6 x 10(6) to 0.7 x 10(6) respectively; (ii) a 48 x 10(3) to 54 x 10(3) mol. wt. nucleocapsid (N) polypeptide; and (iii) two external glycopolypeptides, 72 x 10(3) to 84 x 10(3) mol. w. (G1) and 30 x 10(3) to 40 x 10(3) mol. wt. (G2). Cross-immune precipitation analyses have confirmed that viruses in the Nairovirus genus share antigenic determinants and are antigenically distinct from representative members of th Bunyavirus, Phlebovirus and Uukuvirus genera (Bunyaviridae).