PD 174265 is a potent, cell-permeable inhibitor of the tyrosine kinase activity of the epidermal growth factor (EGF) receptor (IC50 = 0.45 nM). It is effective in cells, blocking tyrosine phosphorylation induced by either EGF or heregulin (IC50s = 39 and 220 nM, respectively). The action of PD 174265 is reversible, which makes it a useful tool for comparative studies with irreversible inhibitors of EGFR kinase activity.