Cells from a spontaneous, invasive, and metastasizing mouse mammary adenocarcinoma (TS/A-pc) were transfected with a retroviral vector containing the mouse IFN-alpha 1 gene. TS/A clones secreting varying amounts of IFN-alpha 1 were isolated and their tumorigenicity was evaluated after s.c. or i.v. injection into immunocompetent BALB/c mice. Almost all of the IFN-alpha-secreting TS/A clones failed to grow in a high percentage of mice or formed small tumors after a long latency time, whereas TS/A-pc or transfection control cells always grew into large s.c. tumors. Rejection was mainly mediated by CD8+ T lymphocytes and partially by polymorphonuclear cells, as demonstrated by selective immunosuppression experiments and histologic and ultrastructural data. After rejection, a significant portion of mice displayed an immune resistance to the subsequent challenge with TS/A-pc. When the metastatic ability of IFN-alpha-secreting clones was compared with that of previously characterized IFN-gamma-secreting TS/A clones, it was found that the expression of IFN-alpha into TS/A tumor cells resulted in a potent inhibition of metastases formation, whereas IFN-gamma expression either did not affect or even enhanced the metastatic behavior of TS/A cells. These results provide strong evidence for the usefulness of IFN-alpha-producing tumor cells for the development of gene therapy strategies and vaccines against metastatic tumors.