ATP and UTP stimulate the proliferation of CD34⁺ HSCs in serum-repleted cultures. CD34⁺ cells (1 × 10⁴) were plated in methylcellulose in the presence of GM-CSF, IL-3, SCF, EPO, and increasing concentrations of ATP (A-D) and UTP (E-G). CFU-GMs (A,E), BFU-Es (B,F), and total CFU-Cs (C,G) were scored after 14 days of culture and the results represent the mean ± SEM of 6 different experiments. The number of colonies in control cultures was 157 ± 18 SEM. In panel D, the stimulatory activity of a fixed dose of ATP (1 nM) was inhibited by the blocking agent apyrase and restored when denatured apyrase was used. ^*P < .05.