Human tau-protein kinase I (TPK-I; also known as glycogen synthase kinase-3beta, GSK-3beta) is a serine/threonine protein kinase. Full-length TPK-I/GSK-3beta was expressed in Escherichia coli as a fusion protein with a 6xHis tag at the C-terminus and was crystallized using the hanging-drop vapour-diffusion method. Prismatic crystals of dimensions 0.4 x 0.2 x 0.1 mm were obtained using 12-15%(w/v) polyethylene glycol 6000 as a precipitant at 278 K. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 82.9, b = 86.1, c = 178.1 A measured at 100 K, diffract to 2.3 A resolution and seem to contain two enzyme molecules per asymmetric unit.