Normal 3- to 4-week old BALB/c mice inoculated with murine sarcoma virus (MSV) develop sarcomas within 7 to 10 days, which generally regress by day 15 to 20. Sublethal irradiation of the recipients, prior to inoculation of MSV, leads to the development of progressively growing tumors and ultimate death of the host within 30 days. Small numbers (1 × 107) of spleen cells from mice whose tumor has regressed, but not similar numbers of cells from age-matched control mice, protect these irradiated mice from developing progressively growing tumors. Pretreatment of the protecting cells with anti-θ antibody and complement, but not with anti-immunoglobulin antibody and complement, abolishes the protective effect.