Dendritic cells (DC) purified from murine spleen or generated in vitro from bone marrow precursors were compared for their respective abilities to stimulate T cell responses and provide tumor protection in vivo. In vitro incubation with synthetic tumor peptide conferred on both DC populations the ability to induce proliferation of tumor-peptide-specific T cells in vitro. Spleen DC were reproducibly about twofold more effective than bone-marrow-derived DC in this assay. Both DC populations could also induce cytotoxic activity in vivo. In vitro cytoxicity assays showed that, while cytotoxic activity induced by immunization with spleen DC was clearly peptide-specific, a high non-specific cytotoxic activity was consistently observed after immunization with bone-marrow-derived DC, whether peptide-pulsed or not. Regardless of such high non-specific activity in vitro, only tumor-peptide-pulsed DC could provide protection against subsequent inoculation of tumor cells. DC not pulsed with tumor peptide were ineffective. We conclude that DC isolated from spleen or generated in vitro from bone marrow precursors are suitable reagents for use in tumor vaccination studies.