Induction of specific antitumor cytotoxic T cell responses was studied in BALB/c mice bearing tumors transfected with a mutant human p53 minigene. We observed that mice were resistant to the induction of peptide-specific CTL as early as 5 days after challenge with a minimal lethal dose of tumor cells, and the cell types responsible for this effect were further characterized. The contribution of CD4+ and CD8+ T cells in this response was studied after peptide-pulsed dendritic cell (DC) immunization. In vitro depletion of CD4+ cells during peptide restimulation reduced the level of specific lysis in control mice, and depletion of CD8+ T cells completely abrogated it. Substitution of CD8+ cells from immunized control mice during restimulation of cells from immunized tumor-bearing mice did not change the level of specific lysis. Substitution of the CD4+ from tumor-bearing mice by CD4+ cells from control mice improved CTL response, although this response did not reach control values. Peptide-pulsed dendritic cells isolated from tumor-bearing mice showed a significantly reduced ability to induce specific CTL in control animals and reduced ability to restimulate immune T cells from control mice in vitro. DC from tumor-bearing mice also had a reduced ability to stimulate control allogeneic T cells. Restimulation of T cells from immunized tumor-bearing mice with DC from control animals, but not from tumor-bearing mice, dramatically increased specific CTL responses to control levels. Macrophages at the same concentration were not able to improve CTL function. Thus, defective antigen presentation by DC appears to be a major determinant for CTL nonresponsiveness to peptide antigens in tumor-bearing mice, and addition of control DC can restore specific lysis. These data provide a basis for new approaches to peptide-based cancer immunotherapy.