T he demonstration of autologous CD8+ CTL with specificity for class I MHC-restricted tumor antigens is a cornerstone of the field of tumor immunology (1, 2). Immunotherapy of malignant diseases is therefore an attractive possibility, but the frequency of immunoreactive T cells is extremely low and variable in the tumor-bearing host (3, 4). The critical challenge now is to augment the frequency of active, tumor-specific lymphocytes, thus translating a trace repertoire into effective immune responses in vivo. Three papers in this issue have begun to address this challenge by using dendritic cells to elicit antigen-specific, tumor-resistant immune responses in mice (5-7).