Extracellular nucleotides enhance the colony growth of G-CSF-primed and steady-state BM CD34⁺ cells stimulated by single cytokines, in presence of EPO, in serum-free cultures. A total of 1 × 10⁴ PB (A-C) or BM (D-F) CD34⁺ cells were plated in semisolid medium under serum-free conditions. Optimal doses of ATP (1 nM) or UTP (10 μM) were added to the EPO-, GM-CSF/EPO-, IL-3/EPO-, and SCF/EPO-stimulated cultures. CFU-GMs (A,D), BFU-Es (B,E), and total CFU-Cs (C,F) were scored after 14 days of culture and the results represent the mean ± SEM of 4 different experiments. ^*P < .05. Cells were also loaded with fura-2/AM as reported in “Materials and methods” and stimulated with 10 μM UTP (G).