UTP exerts its activity on HSCs after short-term exposure and induces the expansion of primitive CD34-derived LTC-ICs. CD34⁺ cells were incubated in liquid culture in serum-free medium, with and without UTP, for up to 24 hours and then plated in methylcellulose. Addition of UTP up to 1 hour induced a remarkable stimulatory activity on CD34-derived CFU-Cs (A). In panel B, 1 × 10⁴ highly purified CD34⁺ cells/mL medium were plated onto irradiated murine stromal cells (M2-10B4) genetically engineered to produce G-CSF and IL-3 with weekly half-medium change. Nucleotides were added to the culture at each medium change (ATP, 1 nM; UTP, 10 μM). After 5 weeks at 37°C in a humidified 5% CO₂ atmosphere, the cells were then evaluated for their LTC-IC content. The results represent the mean ± SEM of 3 different experiments. The addition of extracellular UTP results in the significant expansion of early LTC-ICs. ^*P < .05.