Based on the evidence that CD8⁺; cytotoxic T cells (CTL) precursors do not appear to distinguish between virus-infected cells and viral peptide-puised syngeneic cells, we have developed methods for priming class I MHC molecule restricted CD8⁺; CTL with such peptides without using any adjuvant. We were able to prime in vivo such CTL immunity lasting at least 6 months with a single i.v. injection of syngeneic 2200–3300 red irradiated peptide-pulsed spleen cells, and even more efficiently with a very small number of irradiated class II MHC molecule expressing spienic dendritic cells (DC). No foreign serum source was necessary during the puising. Interestingly, we could not generate significant CTL activity with unirradiated or low dose (<1100 rad) irradiated spleen cells. Because even purified DC required irradiation for optimal activity, because unirradiated B cells did not significantly inhibit the immunization with DC, and because B cell depletion did not substitute for irradiation, we believe that the effect of irradiation is more to determine homing of the cells than to eliminate interference by B cells. Intravenous immunization was much more effective than s.c. or I.p. immunization. CTL generated by this method could kill both peptide-pulsed syngeneic targets and targets endogenously expressing the whole gp160 gene. Moreover, we found that we could prime CD8+ CTL with the minimal 10-residue core peptide (RGPGRAFVTI) for optimal presentation by class I MHC molecules as efficiently as the original p18. These results suggested that DC bearing antigenic peptide may prime antigenspecific CD8+ CTL in vivo. These results offer useful information for development of synthetic peptide vaccines and immunotherapy.