Induction of proliferation and differentiation in response to the addition of recombinant human granulocyte colony-stimulating factor (G-CSF) was studied by both suspension and semisolid cultures in a series of acute myeloid leukemias (AML). Induction of proliferation by G-CSF alone was observed in six of 27 cases of AML. All acute promyelocytic leukemias with the specific chromosomal translocation t(15;17) and one case of myelomonocytic leukemia with balanced chromosomal translocation involving chromosome 17 at band q12q21 were induced to proliferate strongly by the G-CSF. However, contrary to the long-term proliferative effect observed with granulocyte/macrophage colony-stimulating factor (GM-CSF), G-CSF activity can be characterized by its capability to initiate and promote the growth of responding AML cells but not to sustain long-term proliferation. Finally, no terminal differentiation was found, as assessed by morphology, cytochemistry, and cell surface marker analysis. These results indicate that G-CSF may be sufficient to provide a specific signal for induction of a transient proliferation in AML without induction of terminal differentiation. The cells with the highest response are clonal leukemia cells, all bearing a translocation involving the chromosome region 17q12q21 in which the G-CSF gene has been recently located.