There is evidence to suggest that the p120 GAP (GAP), originally described as an inhibitor of p21ras, may also serve as a downstream effector of ras-regulated signal transduction. To determine whether GAP expression is required for the growth of human normal and leukemic hematopoietic cells, we used GAP antisense oligodeoxynucleotides to inhibit it and analyzed the effects of this inhibition on the colony-forming ability of nonadherent, T lymphocyte-depleted mononuclear cells and of highly purified progenitors (CD34+ MNC) obtained from the bone marrow and peripheral blood of healthy volunteers or chronic myeloid leukemia (CML, bcr-abl-positive) patients. The acute myelogenous leukemia cell line MO7, the Philadelphia BV173 cell line, and the acute promyelocytic leukemia NB4 and HL-60 cell lines were similarly examined. GAP antisense treatment inhibited colony formation from normal myelo-, erythro-, and megakaryopoietic progenitor cells as well as from CML progenitor cells. Proliferation of MO7 (growth factor-dependent) and BV173 (bcr-abl-dependent) cells, but not that of NB4 and HL-60 (growth factor-independent) cells, was also inhibited, even though a specific downregulation of GAP was observed in each cell line, as analyzed by either or both mRNA and protein expression. Stimulation of MO7 cells with hematopoietic growth factors increased the expression of GAP as well as the levels of active GTP-bound p21ras. Stimulation of GAP expression was inhibited upon GAP antisense treatment. These data indicate that p120 GAP is involved in human normal and leukemic hemopoiesis and strongly suggest that GAP is not only a p21ras inhibitor (signal terminator), but also a positive signal transducer.