SUMMARYMonoclonal antibodies were prepared from mice immunized with an 18-residue synthetic peptide with an amino acid sequence from a major antigenic sequence involved in the neutralization of type 3 poliovirus. Approximately 250 hybridomas secreted antibodies that reacted with the peptide but not the virus, two antibodies reacted with the virus but not the peptide and no antibody reacted with both. Conversely 26 monoclonal antibodies prepared from mice immunized with type 3 poliovirus and known to be directed against the appropriate sequence on the virus, generally failed to react with the peptide. These results might be expected if only a small proportion of the free or coupled peptide molecules adopt molecular conformations which resemble that of the homologous antigenic site in the virus. Antibodies specific for other antigens occasionally reacted well with the synthetic peptides, indicating that antibodies may bind to peptides of inappropriate sequence. The identification of antigenic sites by the use of synthetic peptides therefore requires considerable caution.We have used indirect biological methods to identify antigenic sites involved in the neutralization of type 3 poliovirus Evans et al., 1983). Three sites have been identified, an immunodominant site at amino acids 89 to 100 on VP1, a site on VP2 and a complex site composed of residues from VP3 and VP1 . These findings are consistent with the published X-ray crystallographic structure of poliovirus (Hogle et al., 1985) and with studies using polyclonal sera ). Other workers have used different approaches to identify antigenic sites on type 1 poliovirus. These include cross-linking of monoclonal antibodies to the virus (Emini et al., 1982), the induction of neutralizing antibodies by synthetic peptides (Chow et al., 1985) and the binding of neutralizing monoclonal antibodies to a VPl-fl-lactamase fusion protein (Van der Werf et al., 1983). In addition the ability of monoclonal or polyclonal antibodies to bind to synthetic peptides in direct ELISA has been used as a criterion for the identification of important antigenic sites in poliovirus type 1 (Emini et al., 1983), hepatitis B virus (Neurath et al., 1984) and Epstein-Barr virus (Dillner et al., 1984) and has been suggested as a general method (Geyson et al., 1984). We have therefore examined the immunological properties of synthetic peptides whose sequences were based on the known immunodominant site of type 3 poliovirus. The findings suggest that the binding of antibodies to synthetic peptides in direct ELISA cannot be regarded as a universally valid approach, and that the results of such studies should be interpreted with caution.The peptides used in these studies were synthesized by the Fmoc-polyamide method (Cambridge Research Biochemicals, Cambridge, U.K. ; Brown et al., 1983) and their sequences are given in Table 1. Synthetic peptide S10a, which contains the amino acid sequence of the major antigenic site on VP 1 of the Sabin vaccine strain of type 3 poliovirus, has been shown to induce ty...