Previously, an epitope recognized by a set of neutralizing monoclonal antibodies directed against the S protein of transmissible gastroenteritis has been identified. This neutralization epitope can be simulated by a single peptide combining residues 380 to 387 and 1176 to 1184 of the S protein; this combination peptide (SFFSYGEI-QLAKDKVNE) was more antigenic than it single constituents. Here we describe the immunogenicity of this combination peptide, in comparison with monomer and tandem peptides of both constituents, and with a cyclic peptide consisting of residues 373 to 398. All antisera, raised in rabbits, bound to the peptide used as immunogen. Only sera that recognized the residues 380 to 387 bound to whole virus. Three of the four antisera with the highest binding titers to whole virus also had neutralization activity. Analysis of the fine-specificity of the antisera with PEPSCAN peptides indicated that the spectrum of antibodies induced by the 380 to 387 sequence depended on the presentation of this sequence in a peptide to the immune system. The nonbinding and nonneutralizing anti-(380 to 387)-sera appeared to contain a limited spectrum of antipeptide antibodies. Furthermore, the lack of neutralization of the antiserum against the combination peptide could be explained by the immunodominance in rabbits of the 1176 to 1184 sequence over the 380 to 387 sequence. These findings demonstrate a few fundamental problems of simulating discontinuous epitopes by single synthetic peptides.