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Protein kinase CK2, which was formerly known as casein kinase II, is a highly conserved protein serine/threonine kinase implicated in the control of cell proliferation through its phosphorylation of regulatory nuclear proteins. The enzyme consists of catalytic (α and (or) α′) subunits and β subunits that modulate the activity of the catalytic subunits. These subunits are arranged in homotetrameric (i.e., α2β2or α′2β2) or heterotetrameric (i.e., αα′β2) complexes. We previously demonstrated using the yeast two-hybrid system that α (or α′) subunits can interact with β subunits but not other α (or α′) subunits. By comparison, β subunits can interact with α (or α′) and with β subunits, suggesting that the protein kinase CK2 holoenzyme forms because of the ability of p subunits to dimerize, bringing two heterodimers (αβ or α′β) into a tetrameric complex. In the present study, we used the yeast two-hybrid system to examine the domains of interactions between the α and β subunits of protein kinase CK2. These studies indicate that the ability of β to interact with α resides within the carboxy-terminal domain of β. By comparison, our studies suggest that individual domains of α are not sufficient for interactions with β.Key words: protein kinase CK2, casein kinase II, yeast two-hybrid system, subunit interaction, signal transduction.