1. 6-Mercaptopurine is a moderate inhibitor of Sarcoma 180 in experiments terminated at 8 days.

  2. Adenocarcinoma E 0771 is moderately inhibited by 6-mercaptopurine, while Adenocarcinoma 755 is profoundly inhibited by this purine antagonist. If treatment is initiated at 24 hours after tumor implantation, growth of 755 is almost completely prevented for 12 days, and growth of 755 may be halted if treatment is initiated after 8 days.

  3. Under certain conditions 6-mercaptopurine appears to exhibit carcinolytic activity against Adenocarcinoma 755 as compared to the carcinostatic activity of 8-azaguanine.

  4. 6-Mercaptopurine inhibits subcutaneous growth of leukemic L1210 and significantly increases life span of mice with this leukemia.

  5. A-Methopterin plus 6-mercaptopurine appears to be no more effective than either drug alone against the solid tumors Sarcoma 180 and Adenocarcinoma E 0771; however, this combination appears to be somewhat more effective than either drug alone against leukemia L1210.

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This work was supported by grants from the American Cancer Society.

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