A panel of 9 mouse hybridomas producing monoclonal antibodies (MAb-2) to determinants of rat MAb 4H5 (MAb-1) was prepared. MAb-1 blocked hemagglutination caused by different alphaviruses. The specificity of paratopes of MAb-2 was studied by competitive enzyme immunoassay and they were divided into 2 groups: 1) interacting with isotypic and/or allotypic determinants of molecules of MAb 4H5 (5 hybridomas) and 2) specific to idiotypic determinants of MAb 4H5 molecules (4 hybridomas). The antigenic structure of paratopes of 3 antiidiotypic MAb (1C2, 1F9, and 6C8) was specific to the idiotopes localized in the active MAb 4H5 center interacting with the hemagglutination domain of VEE virus glycoprotein E2. These antiidiotypic MAb can agglutinate goose red cells, this indicating that the antigenic structure of their paratopes mimicks the functional determinant of the hemagglutination domain of VEE virus glycoprotein E2.