Effect of neutralizing anti-Tpo antibody on rhIL-11– and SF-induced human MK colony formation. CD34⁺ cells were isolated from human BM, assayed in serum-free media for BFU-MK–derived (A) and CFU-MK–derived (B) colonies, and scored as described in Fig 2. The following cytokine concentrations were used: rhIL-11 (50 ng/mL), rhSF (50 ng/mL), and rhTpo (10 ng/mL). The anti-Tpo antibody was added into the cultures at 75 to 100 μg/mL, and the isotype antibody was added at 75 to 100 μg/mL. Each column with bar corresponds to the mean ± SEM of triplicate cultures from an experiment representative of three separate experiments.