Granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood progenitor cell (PBPC) collections are increasingly emerging as the graft of choice in many centers for autologous transplantation, and with increasing frequency for allogeneic transplantation. However, the role of myeloid cytokines in lymphoid function, lymphoid progenitors, and immune-mediated antitumor responses is not known. We studied PBPC collections from normal donors mobilized with G-CSF (10 μg/kg). CD56+CD3- natural killer (NK) cells sorted from PBPC products exhibited a diminished ability to kill tumor targets, were less responsive in acquiring increased cytolysis with interleukin-2 (IL-2), and proliferated less than NK cells from normal unprimed peripheral blood. This abnormality was not explained by a change in phenotype of NK cells normally circulating in the blood after G-CSF administration. We could not demonstrate any direct suppressive effect on normal unprimed NK cell proliferation or cytotoxicity by culture with pharmacologic concentrations of G-CSF. We next evaluated the effects of G- CSF on CD34+ NK cell progenitors. CD34+/CD2+, CD34+/CD7+, and CD34+/CD10+ progenitors were markedly diminished in G-CSF-mobilized PBPC products. CD34+ cells cultured in limiting dilution assays showed a sixfold decrease in NK cell progenitors when derived from G-CSF-mobilized CD34+ PBPCs compared with CD34+ cells derived from unprimed marrow. The finding of decreased NK cell function, inhibited proliferation, and diminished cloning frequency after treatment with G-CSF could be mimicked in vitro by culture of primitive marrow progenitors (CD34+, lineage-negative, HLA-DR-) on stromal layers in the presence of exogenous G-CSF. The findings presented here show that G-CSF administration to normal donors decreases NK cell function and the relative frequency of NK cell progenitors within the CD34+ progenitor population. Overcoming this diminished lymphoid capacity may be important to facilitate early post-transplant immunotherapy. Our in vitro model will be used in future studies to determine the mechanism of the G-CSF-induced suppression of NK cell progenitors, which may occur early in the differentiation process.