SUMMARY L5178Yf cells were labeled with 125I-iododeoxyuridine (125IUdR) and the death of these cells was estimated in normal and preimmunized allogeneic mice by monitoring 125I excretion. It was found that the death rate of the tumor cells injected into mice sensitized to H-2 or a complex of non-H-2 antigens was much greater than that found in nonimmunized control mice. More than 90% of the label was excreted from the primed animals within 2 days. Spleen cells were as effective as tumor cells in sensitizing the mice to H-2 and/or non-H-2 antigens. Host survival closely correlated with the 125I data, i.e., all preimmunized mice survived, while control mice invariably died following tumor inoculation. The response was specific since C57BL/10Sn mice immunized with DBA/2 tissue and challenged with a DBA/2 tumor excreted I25I at an accelerated rate, while the same animals challenged with a C3H tumor did nos. No significant reutilization of the label occurred within the first 3 days of the experiment