Transforming growth factor-β (TGF-β) has growth-stimulating effects on mesenchymal cells and several tumor cell lines. The signaling pathway for this effect is, however, not well understood. We examined how TGF-β stimulates proliferation of MG63 human osteosarcoma cells. Two distinct type I receptors for TGF-β, ALK-1 and ALK-5, were expressed and functional in MG63 cells. Of these two receptors, ALK-5 appears to be responsible for the growth stimulation because expression of constitutively active ALK-5, but not ALK-1, stimulated proliferation of MG63 cells. SB-431542 (0.3 μm), a novel inhibitor of ALK4/5/7 kinase, suppressed TGF-β-induced growth stimulation. DNA microarray analysis as well as quantitative real-time PCR analysis of RNAs from TGF-β-treated cells demonstrated that several growth factors, including platelet-derived growth factor AA, were induced in response to TGF-β in MG63 cells. Gleevec (1 μm) as well as AG1296 (5 μm) inhibited TGF-β-induced growth stimulation of MG63 cells, suggesting that platelet-derived growth factor AA was mainly responsible for the growth-stimulatory effect of TGF-β. We also examined the mechanisms of perturbation of growth-suppressing signaling in MG63 cells. We found that expression of c-Myc, which is down-regulated by TGF-β in many other cells, was up-regulated in MG63 cells, suggesting that up-regulation of c-Myc expression may be the mechanism canceling growth-suppressing signaling of TGF-β in MG63 cells.