Haemopoietic cells from patients with leukaemia and aplastic anaemia were studied in vitro with a standard soft agar tissue culture method. In the leukaemic patients a range of growth patterns was observed, but in most cases the results indicated that leukaemic cells retain the capacity to proliferate in response to the normal physiological neutropoieic regulator, colony-stimulating factor. In some patients, cells were apparently also capable of normal morphological maturation. These findings support the work of others in suggesting that leukaemic cells may be induced to behave normally in vitro, at least in some cases, raising possibilities of radically altered approaches to further therapy. In aplastic anaemia granulocyte-macrophage progenitor cells are greatly diminished.