Four naturally occurring antigenic variants of foot-and-mouth disease virus type A12 were examined for their capacity to be neutralized by a number of monoclonal antibodies (MAb) which recognize different sites on the virus surface. The VP1 coding region of the RNA genome was sequenced and amino acid changes were determined for the variants. One of the neutralizing sites accounted for the differing antigenic properties of the variants and the epitope was mapped to amino acid residues 150-156 of VP1. Another epitope originally thought to occupy a single site in the area of amino acids 168-179 of VP1 was found on all of the variants. Competitive binding assays did not identify the exact binding site for this monoclonal antibody and the results suggest that the epitope may represent a site more complex than a sequential epitope. A polyclonal antiserum to a 13 kDa fragment of VP1 (residues 55-179) was found to have all the virus reactivity associated with sites located between residues 133-164 of VP1. In contrast, an antiserum to VP1 was found to have additional virus binding sites outside of the 133-164 region of VP1.