Subcutaneously established tumors in WKA rats were treated with polycation DEAE-dextran (DEAE-D) and Friend murine leukemia virus (F-MuLV). This idea was based on "xenogenization of tumors," which is defined as the immunologic regression of transplanted tumors in syngeneic rats after artificial infection of tumors with murine leukemia viruses. Regressions of subcutaneously established tumors were induced in 13 of 40 (33%) rats by injection of DEAE-D and F-MuLV. Intratumor injections of DEAE-D and F-MuLV increased the regression of tumors in 7 of 12 (58%) rats as compared to that of tumors in 6 of 28 (21%) rats treated with DEAE-D and F-MuLV by other injection routes. Electron-microscopic and immunofluorescence examinations revealed that tumor cells were infected with F-MuLV and acquired F-MuLV-related surface antigen on the cell surfaces. Therefore, the regression in rats of subcutaneously established tumors by the injection of DEAE-D and F-MuLV may have been due to an immunologic mechanism that may have been the same as the xenogenization of transplanted tumors previously infected with murine leukemia viruses.