Using the approach of peptide transduction domain (PTD)-mediated loading of interleukin-2(IL-2)-activated natural killer (A-NK) cells, tumor-seeking lymphocytes, with prodrug-activating enzymes, we primarily aim to generate a cytotoxic drug selectively within tumors and minimize damage to normal tissues. A-NK cells are able to accumulate selectively at tumor sites. While these cells by themselves possess significant antitumor effect in vivo, we suggest that they can also serve as Trojan horses, by bringing anticancer agents, such as prodrug-activating enzymes, selectively to tumors. We have successfully demonstrated in a mouse model that A-NK cells can be rapidly loaded with prodrug-activating enzymes, such as alkaline phosphatase (AP) and beta-galactosidase (beta-gal), in vitro using enzyme-conjugated peptide PTD5. Upon adoptive transfer into lung-tumor-bearing animals, the loaded A-NK cells are able to bring their cargo of the prodrug-activating enzymes selectively to pulmonary metastases. The targeting of the AP to the tumor tissues is highly specific, since more than a fivefold higher concentration of AP was found in the tumor tissues compared to the surrounding normal lung tissue at 24 h after injection. The approach of transporting prodrug-activating enzymes selectively into tumors clearly shows potential for future targeted chemotherapy. Ongoing studies in our laboratory are evaluating the antitumor efficacy of cellular-dependent enzyme prodrug therapy.