We examined the functional differences between G₀ and G₁ cord blood CD34⁺ cells for up to 24 weeks in serum-free suspension culture, containing Flt-3 ligand, thrombopoietin and stem cell factor. By week 24, there is more than a 1,000-fold difference in granulocyte, macrophage-colony-forming cells (GM-CFC) cumulative production between the two populations, with cultures initiated from G₀ demonstrating an amplification of 1.1 × 10⁵-1.8 × 10⁶ of GM-CFC compared to 45-2.7 × 10³ for the G₁ cells. Cells from the initial G₀ population are able to produce about 250-fold higher numbers of BFU-E than those from G₁ which translates to 3 × 10³-1.1 × 10⁵-fold expansion and 25-390-fold expansion for G₀ and G₁, respectively. This amplification of the progenitor cells is reflected in finding that a greater proportion of the progeny of the G₀ population are CD34⁺, resulting in a 600-fold expansion of CD34⁺ cells at week 8. As in other in vitro systems, total cell expansion is less discriminatory of stem cell behavior than progenitor cells, and there is no significant difference in total cell numbers between G₀ and G₁ cultures with a mean fold expansion of 2 × 10⁷ at 24 weeks.