To define conditions for improved efficiency of retroviral-mediated gene transfer and expression in primate progenitor cells, four rhesus monkeys were treated with a 200 mg/kg intravenous bolus of 5-fluorouracil (5-FU). The kinetics of hematopoietic suppression and recovery were assessed in peripheral blood, bone marrow mononuclear cells, and bone marrow cells fractionated in an albumin density gradient. Bone marrow mononuclear cells were transduced with N2, a retroviral vector carrying the bacterial neomycin phosphotransferase gene (NPT), which confers resistance to the otherwise toxic neomycin analogue, G418. Circulating colony-forming units-granulocyte-macrophage (CFU-GM) disappeared at 2 days. CFU-GM, transducible CFU-GM, CD34+ cells, and the percent of cells in cycle decreased at 3 days in unfractionated bone marrow cells and in a light density population known to be enriched for these progenitors and for stem cells. NPT activity in the light-density fraction, marginally detectable before treatment, disappeared at 3 days as well. At day 7 the CFU-GM plating efficiency, the CD34+ cell content, and the percentage of cells in cell cycle began to increase in the light-density fraction. The NPT assay became faintly positive again but the CFU-GM were not yet transducible, implying that it was an earlier progenitor population that was dividing and differentiating. By day 15, there was a marked rebound in all of the progenitors measured, and transduction efficiency assessed by G418R CFU-GM and NPT assay rebounded to several times pretreatment levels. The data suggest that CFU-GM are optimally transduced at 15 days but that earlier progenitors are more likely cycling and transducible before 5 days, a time when a gene transfer experiment would probably have the best chance to succeed.