The priming of an immune response against a major histocompatibility complex class I-restricted antigen expressed by nonhematopoietic cells involves the transfer of that antigen to a host bone marrow-derived antigen presenting cell (APC) for presentation to CD8+ T lymphocytes. Dendritic cells (DC), as bone marrow-derived APC, are first candidates for presentation of tumor-associated antigens (TAA). The aim of this study was to see whether DC are able to prime in vivo antigen-specific cytotoxic T lymphocytes after exposure to a soluble protein antigen in vitro. Lacking a well-defined murine TAA, we took advantage of beta-galactosidase (beta-gal)-transduced tumor cell lines as a model in which beta-gal operationally functions as TAA. For in vivo priming both a DC line, transduced or not transduced with the gene coding for murine GM-CSF, and fresh bone marrow-derived DC (bm-DC), loaded in vitro with soluble beta-gal, were used. Priming with either granulocyte macrophage colony-stimulating factor-transduced DC line or fresh bm-DC but not with untransduced DC line generated CTL able to lyse beta-gal-transfected target cells. Furthermore, GM-CSF was necessary for the DC line to efficiently present soluble beta-gal as an H-2Ld-restricted peptide to a beta-gal-specific CTL clone. Data also show that a long-lasting immunity against tumor challenge can be induced using beta-gal-pulsed bm-DC as vaccine. These results indicate that effector cells can be recruited and activated in vivo by antigen-pulsed DC, providing an efficient immune reaction against tumors.