Three peptides corresponding to fragments of diphtheria toxin have been synthesized. They include the previously described tetradecapeptide and two structural analogs, the hexadecapeptide and the octadecapeptide. Conjugates of these peptides to proteins or a synthetic carrier have induced in guinea pig protection against the dermonecrotic activity of diphtheria toxin. All of the conjugates were immunogenic when administered either in complete Freund's adjuvant or with N-acetylmuramyl-L-alanyl-D-isoglutamine in aqueous medium. Positive immune response toward the octadecapeptide was obtained in mice as well. In this case, the immunogenic combinations were conjugates with bovine serum albumin administered either in Freund's adjuvant or with the muramyl dipeptide and a complete synthetic conjugate comprising both the octadecapeptide and the muramyl dipeptide covalently attached to a synthetic carrier, multichain poly(DLAla). This last immunogen, which induced the most effective immune response, is a completely synthetic immunogen with built-in adjuvanticity and induces protective antitoxic immunity when administered in a physiological medium.