Epidermal growth factor (EGF) induces cell proliferation in a variety of cell types of binding to a prototype transmembrane tyrosine kinase receptor [1,2].Ligation of this receptor by EGF activates Erk1 and Erk2, members of the mitogen-activated protein (MAP) kinase family, through a Ras-dependent signal transduction pathway [3-5]. Despite our detailed understanding of these events, the exact mechanism by which EGF causes cells to proliferate is unclear. Big MAP kinase (Bmk1), also known as Erk5, is a member of the MAP kinase family that is activated in cells in response to oxidative stress, hyperosmolarity and treatment with serum [6,7]. Here we show that EGF in a potent activator of Bmk1. In contrast to Erk1/2, EGF-mediated by activation of Bmk1 occurs independently of Ras and requires and the MAP-kinase kinase Mek5. Expression of dominant-negative form of Bmk1 blocks EGF-induced cell proliferation and prevents cells from entering the S phase of the cell cycle. These results demonstrate that Bmk1 is part of a distinct MAP-kinase signalling pathway that is required for EGF-induced cell proliferation and progression through the cell cycle.