The Ovalbumin(257-264) CTL epitope on the major coat protein of the filamentous bacteriophage in different antigen formulations was displayed and the immune response in C57BL6/J mice studied. The display of single cytotoxic epitope on the surface of the virion is sufficient to induce priming and sustain long-term major histocompatibility complex class I restricted cytotoxic T lymphocytes response in vivo. The filamentous bacteriophage is a versatile carrier able to display simultaneously either single or multiple epitopes and can elicit a cellular response carrying very little peptide (<1.5 microg).