The influence of GM-CSF on bone marrow cultures from 13 patients with aplastic anemia, MDS and acute leukemia was studied in a short-term suspension culture system. In each case combined cytogenetic and proliferation analyses were performed with respect to the question, whether chromosome aberrations play a role in the in vitro response to GM-CSF and in order to search for stimulating effects on malignant cells. The responsiveness was compared of aplastic and myelodysplastic cultures on the one hand and of leukemic cells on the other. Our results show that myelodysplastic and aplastic cells display a tendency for reduced susceptibility to GM-CSF as compared to healthy controls, while in leukemic bone marrow the response to the growth factor was significantly enhanced, indicating a leukemia-specific response pattern. In the majority of leukemias analyzed, the presence of cytogenetically abnormal cells in cultures with excessive response to GM-CSF can be taken as a proof for stimulation of malignant cells. The significance of these findings for pathogenesis and prognosis in aplastic anemia, myelodysplasia and leukemia is discussed.