The leukocytes of chronic granulocytic leukemia incorporate labeled valine and leucine at a higher rate than normal leukocytes. 6-mercaptopurine causes significant decrease in the rate of amino acid incorporation into cellular protein of leukemic cells. The onset of a sharp decrease in the amino acid incorporation rate by granulocytic leukemic cells occurs only after several days of therapeutic oral doses of 6-mercaptopurine. The decrease in incorporation rate precedes the decrease in circulating leukocytes by several days, indicating that damage to a vital function of these cells occurs before their disappearance from the blood stream. The decrease in the amino acid incorporation rate persists as long as the leukemia is in remission and even after therapy has been stopped; it exists until exacerbation occurs. Increase in incorporation accompanies exacerbation of the leukemic cell count. A possible action of 6-mercaptopurine is its role in interference with amino acid incorporation into cellular protein of chronic granulocytic leukemia cells.