The number of granulocytic progenitor cells (colony forming units in culture: CFUc) in the blood of patients with Down's syndrome was found to be reduced by 73.2% when compared to a group of age and sex matched controls. However, the blood CFUc of the Down's syndrome patients and the controls showed similar sensitivity to hydroxyurea which suggests that the low number of progenitor cells in Down's syndrome is not compensated by a marked increase in their cellular proliferation. The colony size distributions were similar for both the patients and the controls and, in addition, repeated assays at various intervals revealed no marked fluctuation in the number of blood CFUc in either group. The significance of the reduced number of circulating CFUc in Down's syndrome in relation to the known susceptibility of such patients to leukaemia is discussed.