Peripheral blood leucocytes from chronic myeloid leukaemia (CML) patients previously untreated or in relapse are agglutinated by concanavalin A (Con A) at concentrations of 25-400 microgram/ml. Leucocytes from normal subjects, from CML patients in remission and from patients with granulocytosis are not agglutinated by Con A. Normal bone marrow cells which contain immature and mature myeloid cells are not agglutinated either by Con A. These results suggest that an alteration occurs in the cell surface of myeloid cells in CML due to leukaemic change and results in the agglutination of these cells by Con A. If peripheral blood leucocytes of CML patients in relapse are separated into immature and mature fractions, both these fractions are agglutinated by Con A suggesting that this alteration occurs in the cell surface of both the immature and the mature myeloid cells.