Recombinant isolated beta-subunit of protein kinase CK2 is readily phosphorylated by p34cdc2/cyclin B kinase at Ser209 with favourable kinetic constants (Km = 1.7 microM, Vmax = 20 nmol.min-1.mg-1). Two synthetic peptides reproducing the 170-215 and the 206-215 C-terminal fragments of the beta-subunit are also phosphorylated though with tenfold higher Km values (19.5 and 28.0 microM, respectively). In contrast, both the beta-subunit associated with the alpha-subunit to give the heterotetrameric holoenzyme and the native CK2 are not appreciably phosphorylated by p34cdc2. These data suggest that the Ser209 beta-subunit phosphorylation observed in intact cells occurs prior to beta-subunit incorporation into the holoenzyme. The isolated CK2 alpha-subunit is not phosphorylated to any appreciable extent by p34cdc2 kinase. Its catalytic activity is nevertheless increased up to fivefold upon incubation with p34cdc2/cyclin B kinase complex. Such a stimulation of activity is comparable to that induced by the beta-subunit and it is paralleled by a 40% decrease of p34cdc2/cyclin B catalytic activity. Similar to beta-subunit, p34cdc2/cyclin B also protects the alpha-subunit against thermal inactivation. CK2 holoenzyme is also stimulated by p34cdc2/cyclin B, albeit less dramatically than the isolated alpha-subunit. Such an effect is also evident with CK2 holoenzyme reconstituted with a mutated beta-subunit lacking the p34cdc2 phosphorylation site and it is not accompanied by any appreciable phosphorylation of either the beta or the alpha-subunit. These data indicate that in vitro CK2 alpha-subunit interacts with and is activated by p34cdc2/cyclin B kinase by a mechanism that does not imply the phosphorylation of CK2.