AbstractThe vasculature of a poorly immunogenic, highly metastatic transplantable fibrosarcoma (T‐241) maintained in the femoral muscle of C57BL/6J mice was perfused. This permitted collection of tumor cells which had invaded into the tumor vascular channels (ie, metastasizing tumor cells). Also collected as a separate population were tumor cells from the primary tumor mass. Immunization was carried out with these cell populations in conjunction with BCG and the effect on the growth of primary tumor and metastatic rate was evaluated following rechallenge with unfractionated tumor cells. The rate of tumor growth at the primary site was not affected by any of the immunization schedules. However, immunization with venous effluent cells (metastasizing tumor cells) and BCG was two times more effective in reducing the number of pulmonary metastases than immunization using tumor cells isolated from the primary tumor mass. Passively transferred spleen cells from donors immunized with the cell populations listed above had exactly the same effect, that is, no effect on the growth of the primary tumor, but a dramatic reduction in the metastatic rate when effluent tumor cells were used to immunize cell donors. The data point to an antigenic heterogeneity with this particular transplantable tumo