The activity of the catalytic alpha subunits of protein kinase CK2 is modulated by interaction with the regulatory beta subunits. In order to define the domains involved in intersubunit contacts, we have applied the two-hybrid system, which a is yeast-based genetic method for the detection of protein-protein interactions in vivo. The data demonstrate that the alpha and beta subunits interact with each other and that the beta subunits, but not the alpha subunits, are able to self-associate. This suggests that the beta subunits play a bridging role in the architecture of the CK2 holoenzyme by linking two alpha:beta heterodimers into a tetrameric complex. Analysis of truncated alpha and beta subunits was used to delimit the subregions necessary for complex formation. The data reveal that the beta subunit is modular in structure, with the two fully separable domains involved in homomeric beta:beta and heteromeric alpha:beta interactions, respectively. Also, beta subunits lacking the autophosphorylation sites in the N termini are able to associate with both the alpha and beta subunits. Furthermore, we find that the N terminus and the evolutionarily less conserved C terminus of the alpha subunit are dispensable for establishing heterodimeric alpha:beta structures.