A panel of five neutralization-resistant escape mutant (EM) viruses was used to investigate the neutralization determinants of the U.S. prototype strain of bluetongue virus serotype 10 (BTV-10). The phenotypic properties of each EM virus were characterized by neutralization and immuneprecipitation assays with a panel of four monoclonal antibodies (MAbs). These MAbs were used to select the various EM viruses and together the MAbs define four distinct neutralizing epitopes on the prototype strain of BTV-10 (Heidner, H.W., Rositto, P.V., and MacLachlan, N.J., Virology 176, 658-661 (1990)). Sequencing of the L2 gene identified mutations responsible for the altered phenotypic properties exhibited by each EM virus. The L2 gene encodes BTV outer capsid protein VP2 which is responsible for virus neutralization. Four amino acids in three distinct regions of VP2 are critical to expression of the epitopes recognized by the MAb panel. Both amino acid 208 and 211 can affect the binding of MAb 039 and MAb 045, amino acid 327 affects binding of MAb 041, and amino acids 327 and 402 cooperatively interact to affect binding by MAb 034. The location of two of these critical regions on VP2 of BTV-10 is identical to two of those which affect neutralization of Australian BTV-1, despite the fact that these two viruses are antigenically distinct and have divergent L2 gene sequences (Gould, A.R., and Eaton, B.T., Virus Res. 17, 161-172 (1990)). The four individual neutralizing epitopes on VP2 of BTV-10 are interactive (Heidner, H.W., Rositto, P.V., and MacLachlan, N.J., Virology 176, 658-661 (1990)) and at least two are conformationally dependent.