The objective of this study was to evaluate the cycling status of cord blood (CB)-derived colony-forming cells (CFC) and long-term culture-initiating cells (LTC-IC), and their recruitment into the S-phase of the cell cycle. By using the cytosine arabinoside (Ara-C) suicide approach, we found that only small proportions of both CFC and LTC-IC were in the S-phase of the cell cycle. These estimates were confirmed by flow cytometric DNA analysis, which showed that 96 +/- 2% of CB-derived CD34+ cells were in G0/G1 and only 1.6 +/- 0.4% in the S-phase. Staining of CD34+ cells with an antistatin monoclonal antibody, a marker of the G0 phase, indicated that among CD34+ cells with a flow cytometric DNA content typical of the G0/G1 phase 68 +/- 7% of cells were in the G0 phase of the cell cycle. Incubation (24 h) with interleukin 3 (IL-3), recombinant human stem cell factor (SCF) and granulocyte colony-stimulating factor (G-CSF) significantly increased the proportion of cells in the S-phase for both CFC and LTC-IC without inducing any loss in numbers. Flow cytometric DNA analysis also showed an increase in CD34+ cells in the S-phase upon continuous exposure to these cytokines. Our findings indicate that: (i) very few CB-derived CFC or LTC-IC were in the S-phase of the cell cycle; (ii) a substantial amount of CD34+ cells with a flow cytometric DNA content typical of the G0/G1 fraction was cycling, as found in the G1 phase of the cell cycle; and (iii) 24-h incubation with IL-3, SCF and G-CSF could drive a proportion of progenitor cells into the S-phase without reducing their number. These data might be useful for gene transfer protocols and the ex vivo expansion of CB-derived progenitor cells.