Regression of myelocytic leukemia in rats after hypophysectomy.

A standard series of five intravenous injections of N-nitroso-N-methylurea (NMU) elicited selective patterns of neoplasms in male rats of the Sprague-Dawley strain; the neoplastic pattern was related to the age of the recipients. When the injections of NMU were started in weanling male rats, a specific triad of neoplasms developed that consisted of mammary carcinoma, primary myelocytic leukemia, and ear duct cancer. When the injections of NMU were started in males at age 100 days, a specific dyad of neoplastic growths was evoked that consisted of primary myelocytic leukemia and ear duct cancer, whereas mammary cancer and other neoplasms were not evident. Secondary myelocytic leukemia and myelocytic sarcoma were produced when blood from the foregoing rats with advanced primary myelocytic leukemia was injected in a subcutaneous site in newborn allogeneic rats. In a proportion of animals, primary and secondary myelocytic leukemia and myelocytic sarcoma underwent dramatic and continued regression after hypophysectomy.