A 74,000 molecular weight glycoprotein was purified from the plasma of a patient with chronic myelogenous leukemia in blast crisis. Monoclonal antibodies were produced in the mouse and used to characterize this protein. It was shown to contain p15E antigenic determinants and portions of a reverse transcriptase. The level of this protein was found to be elevated in leukemic patients with high white blood cell counts and also in some patients with other hematopoietic disorders as compared to the level measured in normal individuals. The level of the protein was strongly reduced in acute leukemia patients after intense chemotherapy treatment. We tentatively conclude that this protein is of endogenous retroviral origin and perhaps regulates hematopoietic tissues.

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