In this manner, those proteins to which neutralizing antibodies are directed have been determined, e.g., VP1 in poliovirus (Osterhaus et al., 1981b; Thorpe et al., 1982) , VP2 in Bluetongue virus Letchworth and Appleton, 1983) , and gp 56 in Venezuelan equine encephalitis virus (Roehrig et at., 1982a) .