We cloned the streptolysin O gene from the Streptococcus pyogenes genome and tested the possibility of using it as an anticancer reagent. Transient transfection of the streptolysin O gene efficiently killed 293T cells after 12 hours of transfection as determined by lactate dehydrogenase release and propidium iodide uptake. No caspase activity was observed and necrosis was prominent during streptolysin O-induced cell death. Biochemical analysis of streptolysin O protein revealed that the deletion of only 5 amino acids from the COOH-terminal region of streptolysin O, which is essential for cholesterol binding activity, abolished its cell-killing activity, whereas the NH₂-terminal region was more resilient, i.e., up to 115 amino acids could be deleted without changing its cell-killing activity. We generated a streptolysin O-expressing adenovirus and injected it into human cervical cancer cell–derived tumors grown in a nude mouse model. Twenty-one days postinjection, the average size of tumors in the streptolysin O adenovirus–injected group was 29.3% of that of the control PBS-treated group. Our results show that the genes of pore-forming toxins, like streptolysin O protein, have the potential to establish a novel class of suicide gene therapeutic reagents. [Mol Cancer Ther 2006;5(6):1610-9]