The ability of Tpo and KL to promote viability of LTC-IC. A total of 525 CD34⁺CD38⁻ BM cells were seeded for each group at a density of 1 cell per well in 10 μL serum-depleted medium in stroma-free cultures. As a control of the initial content of LTC-IC, cells were harvested directly after seeding and transferred to irradiated preestablished human BM stroma. After 116 hours in the presence of Tpo, KL, or medium alone, cells were examined microscopically and wells containing 2 or more were excluded. The remaining wells were harvested, pooled, and divided into 5 wells containing irradiated stroma. The cultures were maintained as described in the Materials and Methods and after 5 weeks the content (adherent and nonadherent cells) in each well was transferred to methylcellulose cultures and the number of CFC was evaluated after an additional 12 to 14 days in culture. The results represent the means (+SEM) of total number of CFC formed by 525 seeded CD34⁺CD38⁻ cells per group in each of four individual experiments.