Protein kinase CK2 is a tetramer composed of two alpha catalytic subunits and two beta regulatory subunits. A C-terminal truncated form of the beta subunit has been overproduced in Escherichia coli and purified to homogeneity. Two crystal forms of the truncated protein which diffract to at least 2 A resolution have been obtained. Form I belongs to the monoclinic space group P21, with unit-cell parameters a = 49.9, b = 92.9, c = 53.7 A, beta = 96.3 degrees, and yields plate-like crystals. Form II belongs to the tetragonal space group P42212, with unit-cell parameters a = 132.19, b = 132.19, c = 63.79 A, and produces rod-shaped crystals. Both crystal forms have a functional dimer in the crystal asymmetric unit.