NCA, a normal colon and granulocyte antigen, which has been found in large amounts in myelocytes and metamyelocytes and in smaller amounts in neutrophil granulocytes, was studied in 50 CML patients in various stages of the disease. Radioimmunoassay was used to demonstrate NCA in serum. Untreated CML patients had a mean level of 732 μg NCNI, poorly controlled patients 421 μg/l and well-controlled patients 160 μ/I. These valuer differ significantly from the mean of healthy persons, which was 71 μg NCN/I. The serum NCA levels were related to the number of maturing myeloid cells in blood, and to the clinical course in the chronic phase of CML In blast crisis low values with a mean of 109 μg NCNI was found. Patients with ANLL had a low mean level, 50 &I. Low levels of NCA could not be attributed to antibodies to NCA. NCA is a normal myeloid differentiation antigen. Despite this, its Occurrence in serum in leukemic patients differs from normai. This probably has to do with the abnormal amount as well as the release of NCA by leukemic maturing myeloid cells.