Preincubation of CD34⁺ cells with antisense oligonucleotides to the Smad5 gene reverses the inhibitory effects of TGF-β1 on human hematopoietic colony formation. CD34⁺BM cells were preincubated with oligonucleotides to the Smad5 gene and plated at a concentration of 2 × 10³/mL in the presence of EPO (5 U/mL), IL-3 (100 ng/mL), GM-CSF (100 ng/mL), and SCF (100 ng/mL) in a methylcellulose culture assay or at 2 × 10⁴/mL in the presence of IL-3 (100 ng/mL) in a fibrin-clot assay system. TGF-β1 was added at 2 ng/mL. The results are expressed as the mean ± SEM of data taken from three separate experiments performed in duplicate. * P ≤ .5 when compared with control values. Control CFU-GM = 63.6 ± 8.0 colonies, control BFU-E = 61.5 ± 8.4 colonies, control CFU-MIX = 21.0 ± 3.6 colonies, and control CFU-MK = 26.0 ± 5.0 colonies. (▪) CFU-GM; (□) CFU-E; (▩) CFU-MIX; (□) CFU-MK.