Studies by a number of investigators have focused on inducing tumor-specific immunity as a therapeutic approach to cancer. Successful immunotherapeutic strategies have involved localized treatment with immune-active adjuvants, systemic administration using cytokines such as interleukin-2, and the use of whole tumor cells or tumor cell fragments as vaccines. With an increasing understanding of the requirements for the development of an immune response, immunotherapeutic strategies have focused on providing mechanistic requirements, such as tumor or accessory antigen expression and cytokine-based" immune help." Recent preclinical studies have shown that ex vivo cytokine gene transfection of tumor cells and their use as vaccines result in the enhanced development of antitumor immunity and in some cases can be used to successfully treat pre-existing tumors. Studies from our laboratory have explored the use of vaccinia virus recombinants to directly transfect tumor cells in situ with cytokine genes as a strategy for enhancing the development of antitumor immunity. We have demonstrated that vaccinia virus recombinants are highly efficient in transfecting a wide range of murine and human tumors in vitro and can be used with similar effects in in vivo murine models. In addition, we have found that vaccinia virus productively infects human melanoma cells following intratumoral injection in patients with accessible lesions. In situ transfection is highly efficient, and therapy with increasing doses of virus is safe with only minor side effects. The results of our studies support the use of cytokine-encoding recombinant vaccinia virus vectors for in situ transfection in patients with cancer.