1. A new antimetabolite, 6-mercaptopurine, has been shown to produce good clinical and hematologic remissions in fifteen out of forty-five children with acute leukemia. Another ten showed partial remissions and clinical improvement.

2. Remissions in adults with acute leukemia have occasionally been brought about by 6-mercaptopurine, and in a few cases it has produced temporary remissions in both the early and the late stages of chronic myelocytic leukemia.

3. The compound has been effective in some children whose disease was resistant to the folic acid antagonists, as shown by the fact that out of twenty-four children with acute leukemia whose disease had been proved to be resistant to amethopterin, five had good clinical and hematologic remissions and five had partial remissions with some marrow and clinical improvement. Some benefit was seen in eight out of eighteen patients whose disease was resistant to ACTH and cortisone.

4. In children the daily oral administration of 2.5 mg./Kg. rarely caused toxic manifestations, but continued therapy at this dose in adults or at higher levels in children occasionally produced bone marrow depression or gastrointestinal symptoms.

5. There is evidence that the therapeutic resistance of the acute leukemias to 6MP develops somewhat more rapidly than it does to the folic acid antagonists but there is, as yet, no laboratory or clinical evidence of cross resistance between these two types of antimetabolites.

6. In a total of thirty-five patients with lymphomas and miscellaneous carcinomas and sarcomas, 6MP did not produce any definite clinical improvement at doses which produced hematologic toxicity.

7. Although 6-mercaptopurine acts as a purine antagonist in certain forms of bacteria, the exact mechanism of its action in leukemia is at present unknown. Since its mode of action appears to differ from that of other agents previously employed clinically in the treatment of leukemia, this compound would appear to be of fundamental as well as practical interest.