Vaccination with dendritic cells (DCs) pulsed with tumor antigen peptides has shown promise in the treatment of melanoma. Interleukin (IL)-12 production by DCs is a key component for their efficacy. Murine studies have shown that IL-12 promotes potent antitumor immunization when coadministered with peptides loaded onto other class I MHC+ cells, thus bypassing the need to use DCs. The easiest cell source to obtain in large quantity from human patients is peripheral blood mononuclear cells (PBMCs). A Phase I clinical trial was thus performed in patients with metastatic melanoma using immunization with autologous PBMCs pulsed with a MAGE-3 or a MelanA peptide, coadministered with various doses of recombinant human (rh)IL-12. Patients receiving low-to-moderate doses of rhIL-12 developed increased specific CD8+ T-cell responses. Of the eight patients showing increased immunity, six had evidence of clinical activity, with one complete, one partial, one minor, and three mixed responses observed. In two patients with mixed responses, growing tumors were found to lack expression of the antigen used to immunize. Thus, vaccination with peptide-pulsed PBMCs plus rhIL-12 induces specific immunity and has clinical activity, without the need to generate DCs. Outgrowth of antigen-negative tumors argues for the future development of polyepitope vaccines.