The incidence of elevated serum levels of immunoreactive calcitonin (CT) in human myeloproliferative and lymphoproliferative disorders was investigated. On the basis of twice the normal range, about 45% of patients with acute leukemia and blast crisis of chronic myelocytic leukemia (CML) showed elevated serum levels of CT. Markedly elevated levels (> 1,000 pg/ml) were only found in this group. Since immunoreactive CT dropped to normal or only slightly elevated levels in remission and increased again before or during relapse, serum CT levels seem to reflect the activity of the disease. However, in patients with chronic leukemia, Hodgkin's and non-Hodgkin's lymphoma, a lower incidence and only slightly elevated serum values were found. In addition, the molecular weight of the proteohormone in serum specimen and cell extracts was investigated by gel chromatography. Besides physiological CT, different high-molecular weight forms of the hormone could be demonstrated in serum and in cell extracts. Extracts of leukemic cells revealed higher molecular forms only. It is suggested that the proteohormone is ectopically produced by leukemic cells.