PURINETHOL (mercaptopurine) is indicated for maintenance therapy of acute lymphatic (lymphocytic, lymphoblastic) leukemia as part of a combination regimen. The response to this agent depends upon the particular subclassification of acute lymphatic leukemia and the age of the patient (pediatric or adult). Mercaptopurine is not effective for prophylaxis or treatment of central nervous system leukemia. Mercaptopurine is not effective in acute myelogenous leukemia, chronic lymphatic leukemia, the lymphomas (including Hodgkins Disease), or solid tumors. Contradictions: Mercaptopurine should not be used in patients whose disease has demonstrated prior resistance to this drug. In animals and humans, there is usually complete cross-resistance between mercaptopurine and thioguanine. Mercaptopurine should not be used in patients who have a hypersensitivity to mercaptopurine or any component of the formulation.