Normal serum and serums from patients with acute and chronic leukemia were assayed for granulocyte colony-inducing activity with human bone marrow cells. Serum from untreated acute leukemia, but not from the other patients, showed about normal inducing activity at low serum concentration and lower than normal activity at high concentration. This suggests that serum from patients with acute leukemia contained an inhibitor for colony formation. Serums from chronic myeloid leukemia were in about the same range as normal, whereas serums from chronic lymphocytic leukemia showed the highest colony-inducing activity.

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