Four antigenic sites of the E2 glycoprotein of transmissible gastroenteritis virus were defined by competitive radioimmunoassays of monoclonal antibodies (MAbs). Here, we describe the localization of these sites by testing the antigenicity of protein fragments and prokaryotic expression products of E2 gene fragments, and by sequencing of MAb-resistant (mar) mutants. Partial proteolysis of purified E2 protein allowed the isolation of a 28K fragment recognized by both site A- and site C-specific MAbs. An antiserum against this fragment bound to a synthetic peptide containing residues 1 to 18 and to an expression product containing residues 1 to 325. The same expression product was recognized by site C-specific MAbs. These data indicate that residues within the sequence 1 to 325 contribute to site C and possibly also to site A. Sequencing of mar mutants that escaped neutralization by site A-specific MAbs indicated that residues 538 and 543 also belong to site A. The binding of site-specific MAbs to expression products led directly to the localization of sites B and D, between residues 1 to 325 and 379 to 529, respectively. The first 37% of the polypeptide chain of E2 appears to be more immunogenic than the rest of the sequence.