The natural cytotoxic activity in vivo can be evaluated by measuring in vivo tumor cell destruction after injecting mice with 125IUdR-labeled tumor cells, measuring their total body radioactivity and calculating the % radioactivity lost. We have studied the in vivo destruction of 125IUdR-labeled L1210 leukemic cells by B10.D2 mice previously immunized 4 times with heavily irradiated L1210 leukemic cells. Mathematical analysis of our results indicates that the radiolabel loss on day 1 is similar in normal and immunized animals, but that it stays greater over the following days in immunized animals, indicating that the difference between the two groups is not the extent of the initial cell destruction but the durability of the response. There is a good correlation between the eventual survival, the % of 125IUdR lost and the number of tumor cells present in the peritoneal cavity three hours after their injection. Such a methodology provides a very early prediction of the survival of each mouse, thus identifying animals with a poor prognosis.