The extracellular signal-regulated kinase ERK2, a member of the protein kinase superfamily, phosphorylates a variety of cellular proteins in response to extracellular signals. ERK2 expressed in Escherichia coli as a fusion protein with the sequence Ala-His6 at the N terminus has low basal activity and very low levels of phosphate incorporation, but can be fully activated. The Ala-His6 ERK2 as expressed in the unphosphorylated form has been crystallized in space group P2(1). The cell constants are a = 49.32 A, b = 71.42 A, c = 61.25 A, and beta = 109.75 degrees, and the crystals diffract to better than 1.8 A resolution.