Previously, we reported that the major stress-inducible heat shock protein 70 (Hsp70) acts as a recognition structure for natural killer (NK) cells, if localized on the cell surface of tumor cells. Incubation of purified NK cells with low-dose interleukin (IL)-2 (100 IU/mL) plus recombinant Hsp70-protein or the immunogenic 14-mer Hsp70-peptide TKDNNLLGRFELSG450–463, termed TKD (2 μg/mL), enhances the cytolytic activity against Hsp70 membrane-positive (CX ) but not against Hsp70-negative (CX−) tumor cells. Here, we show that the cytolytic activity against Hsp70-positive tumor cells is inducible by incubation of unseparated peripheral blood mononuclear cells (PBMNC) with low-dose IL-2 plus TKD. Cell sorting experiments revealed that within the PBMNC population CD94 /CD3− NK cells, and not CD94−/CD3 T cells, mediate the cytotoxic activity against Hsp70-positive tumor cells. The antitumoral effect of PBMNC stimulated either with IL-2 plus TKD or with IL-2 alone was assessed in tumor-bearing severe combined immunodeficiency/beige mice. A single intravenous (iv) injection of 40 × 106 IL-2 plus TKD-stimulated PBMNC (containing 5.2 × 106 NK cells) on day 4 results in a 60% reduction in tumor size, from 3.89 g to 1.56 g. In contrast, the adoptive transfer of the identical amount PBMNC stimulated with low-dose IL-2 only (containing 4.4 × 106 NK cells) reduces the tumor size only less than 10% (3.64 g). A phenotypic characterization of the excised tumors revealed that predominantly Hsp70-positive tumor cells were eliminated by TKD-activated PBMNC. Kinetic studies demonstrate that the in vivo cytolytic capacity of TKD-stimulated PBMNC is dependent on the effector to target cell ratio. An iv injection of effector cells on day 1 or 2 after tumor cell inoculation results in significantly smaller tumors (0.77 g or 0.89 g) on day 21 as compared with mice that were immunoreconstituted on day 4 or 8 (1.39 g or 2.23 g). The tumor size of nonimmunoreconstituted control animals was 3.55 g.