Three permanent human leukaemia cell lines, designated EW2, LG3 and MS6, were established from bone marrow aspirates of a patient with acute myelomonocytic leukaemia (EW2), acute monocytic leukaemia (LG3) and acute myeloblastic leukaemia (MS 6). In vitro all lines exhibited a myelomonocytoid marker profile in terms of classical staining reactions and cell-surface antigen structure. In supernatants and extracts of EW2 and MS 6 cells immunoreactive human calcitonin was found in raised levels. No significant levels of immunoreactive human calcitonin were found in supernatants of LG3 cells, while extracts of LG3 cells and the patients sera at diagnosis contained high levels, suggesting the in vitro selection of a non-secreting clone of leukaemic cells. Gel-chromatography showed several peaks of ihCT with higher molecular weight than normal human calcitonin, suggesting the production of precursor molecules. In addition to ihCT raised levels of immunoreactive calcitonin gene related peptide (ICGRP) was found in supernatants of EW2. These data support the concept of ectopic immunoreactive human calcitonin production by leukaemic cells.