We have achieved efficient transduction of tumour metastases in vivo by the vascular delivery of retroviral producer cells. Experimental liver metastases in mice were created by intrasplenic injection of tumour cells into the portal venous circulation. Following the establishment of micrometastases, delivery of retroviral producer cells by the same route with a vector containing the Escherichia coli β–galactosidase (lacZ) gene demonstrated selective in vivo gene transfer to tumour deposits. By this approach, two retroviral producer cell lines encoding cytokines (IL–4 and IL–2) directed tumoricidal inflammatory responses to established metastases. Cytokine gene targeting inhibited metastasis formation and caused significant overall reduction in tumour burden. These results suggest a novel therapeutic approach for the treatment of disseminated cancer.