In mice bearing a benzypyrene-induced fibrosarcoma tumor, the survival was determined of intravenously injected tumor cells at various intervals after previous immunization of experimental animals by induction and subsequent excision of a transplanted primary tumor in the soft tissues of the leg. Seven days after induction of a transplanted primary tumor, I.V. tumor cells produced fewer pulmonary metastases in immunized mice than in nonimmunized mice. When tumor cells were incoulated I.V. immediately following amputation of the transplanted primary tumor, the number of pulmonary metastases in the immunized and nonimmunized animals were similar; however, 6 hours, 7 days and 14 days after primary tumor excision, I.V. inoculated tumor cells produced a higher incidence of lung metastases in the immunized than in the nonimmunized mice. This increased survival of I.V. tumor cells following excision of the transplanted primary tumor may have relevance to the development of metastases after eradication of certain primary tumors in humans.