A relatively short peptide can be used as an immunogen to elicit antibodies that are capable of reacting with full-length proteins containing the peptide's amino acid sequence. Such peptides are frequently represented in the primary sequence of a protein. The antibodies elicited by these peptides are directed against a specific region of the protein chosen in advance by the investigator and so have a predetermined specificity. In basic research, such antibodies are useful in identifying the gene product of an open reading frame, purifying enzymatically active proteins by immunoaffinity techniques, and investigating protein folding domains and immunogenic structure and function. In medicine, since some peptides are capable of eliciting antisera that can bind to and neutralize virus, the antibodies may be reagents for passive vaccination, antitoxin therapy, and targeted immunotherapy of neoplasia. The peptides themselves may serve as the basis for safe, chemically defined vaccines.