The inhibitory effect of TGF-β1 on hematopoietic colony formation is more profound than that exhibited by TGF-β2. CD34⁺ BM cells were preincubated with oligonucleotides to the Smad5 gene and plated either at 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 and TGF-β2 were both added at 5 ng/mL. The results are expressed as the mean ± SEM of total colony numbers. The data was taken from three separate studies performed in duplicate. *P ≤ .5 when compared with control values.