A-77-01 is a potent inhibitor of the TGF-β1 receptor ALK5 (IC50 = 25 nM). It has no effects on BMP-regulated transcriptional activity or MAPK pathways. A-77-01 efficiently prevents the growth inhibitory effects of TGF-β on lung epithelial Mv1Lu cells. It completely blocks phosphorylation of SMAD2 by TGF-β in human keratinocyte HaCaT cells at 1 µM. A-77-01 prevents morphological and gene expression changes characteristic of epithelial-to-mesenchymal transition in NMuMG cells treated with TGF-β.