This paper demonstrates the potential for utilizing the plant enzyme, horseradish peroxidase (HRP), in a gene-directed enzyme prodrug therapy context. Human T24 bladder carcinoma cells transfected with a mammalian expression vector containing the HRP cDNA were selectively sensitized to the nontoxic plant hormone, indole-3-acetic acid (IAA). The HRP/IAA-induced cell kill was effective in normoxic and anoxic conditions. The activated drug is a long-lived species able to cross cell membranes, and cell contact appears not to be required for a bystander effect to take place. These preliminary results suggest that the delivery of the HRP gene to human tumors followed by IAA treatment may provide a novel cancer gene-directed enzyme prodrug therapy approach, with potential to target hypoxic cells. Cancer Gene Therapy (2000) 7, 1414–1420