Il-4 is a highly pleiotropic cytokine which induces cytotoxic activity when present at the tumor site. Death of tumor cells probably depends on the appearance of an inflammatory infiltrate composed of eosinophils and macrophages. Regression of established tumor masses has been readily observed upon direct intratumor implantation of cells which constitutively produce high amounts of Il-4. We now report a similar potent anti-tumor effect achieved via direct gene transfer, i.e. by injecting Il-4 DNA complexed with cationic liposomes into B16(F10) melanoma tumor in vivo.