We compared the cell cycle status and expression of mRNA for the amphotropic retroviral receptor in hematopoietic stem cells isolated from bone marrow and cytokine mobilized peripheral blood. CD34+ cells from six normal volunteers were enriched by immune selection from steady-state bone marrow and granulocyte colony-stimulating factor (G-CSF) mobilized peripheral blood (10 microg/kg/day for 5 days). Cell cycle status of the phenotypically primitive CD34+CD38- hematopoietic stem cell population was analyzed using a four-color flow cytometry technique that distinguished the G0, G1, and S/IG2/M phases of the cell cycle. Semiquantitative reverse transcriptase-polymerase chain reaction was performed to measure mRNA expression of the amphotropic retroviral receptor. Peripheral blood hematopoietic stem cells had 2.6-fold more cells in the G1 phase of the cell cycle compared to steady-state bone marrow. Furthermore, lineage CD34+CD38- cells from G-CSF mobilized peripheral blood had a fourfold higher level of amphotropic retrovirus receptor mRNA. In conclusion, we found that CD34+ CD38- hematopoietic stem cells isolated from G-CSF mobilized peripheral blood differ from those isolated from steady-state bone marrow in that a significant proportion have entered the G1 phase of the cell cycle and express higher levels of amphotropic receptor mRNA. These biologic properties are consistent with the reported rapid recovery of hematopoietic function following transplantation with peripheral blood hematopoietic stem cells and make these cells a preferred target for retroviral-based gene transfer.