IL-10 bidirectionally modulates growth and proB cell formation from Lin⁻Sca-1⁺c-kit⁺ progenitor cells. A total of 1,000 Lin⁻Sca-1⁺c-kit⁺ cells was plated in complete IMDM in the presence of FL + IL-7 + IL-10. After 7 (A) and 10 (B) days incubation, 10% of the resulting cells were replated per well in round bottomed 96-well plates in FL + IL-7 + IL-10 in the presence of a neutralizing IL-10 antibody (anti-IL–10) or an isotype-matched control antibody (ctr ab). After 24 hours of incubation, wells were pulsed with 1 μCi [³H]-TdR for an additional 12 hours incubation, at which time samples were harvested and counted in a beta-counter as described in the Materials and Methods. The results represent the mean cpm (SEM) from three independent experiments. In each experiment, each group was performed in triplicate.