1. Two patients with acute leukemia had considerable decreases in leukemic cells in the peripheral blood as well as reduction in size of spleen and leukemic masses after 10 injections of 3H-TDR given over a 5-day period. Each injection was 0.25 µc./Gm. body weight.
2. The pertinent aspects of cytotoxic effects of 3H-TDR are reviewed.
3. The radiation doses delivered to the nucleus are estimated from autoradiographic data.
4. Evidence is presented for the observed effects being due to 3H-TDR.