SD-208 is an inhibitor of TGF-β signaling in hematopoietic cells. Leukemic cells (K562 and KG-1) and primary hematopoietic progenitors at the colony-forming unit–erythroid stage of maturation (CFU-E) were treated with TGF-β1 (20 ng/mL) in the presence and absence of SD-208 (.5 μM) and assessed for smad2 phosphorylation by immunoblotting. SD-208 pretreament (1 hour) led to attenuation of activation/phosphorylation of smad2 (A-C). Bone marrow stroma–derived cells (HS-5) were transfected with plasmids expressing smad binding 3TP-luciferase and β-galactisidose (transfection control) and stimulated with TGF-β1 in the presence and absence of SD-208 (dose .5 μM). TGF-β1–induced control-normalized luciferase activity was potently inhibited by SD-208. A kinase-null mutant of TGF-β receptor I (TBRI-KR) was used a positive control (D). Error bars represent SEM.