Binding of p53 to cellular and viral proteins seems to be implicated in the regulation of its growth suppressor or oncogenic activity. Casein kinase II is among the proteins which tightly associate with p53 and which phosphorylate p53 at a C-terminal amino acid residue. In the present study we demonstrate that immunopurified full length p53 as well as a C-terminal p53 fragment which comprises the 130 C-terminal amino acids of authentic p53 is phosphorylated by the casein kinase H holoenzyme but not by the catalytic alpha-subunit of casein kinase II alone. Furthermore, we were able to show that the C-terminal p53 fragment binds to the regulatory beta-subunit of casein kinase II but not to the alpha-subunit. Binding of the C-terminal p53 fragment to the beta-subunit is not severely influenced by phosphorylation of p53 by casein kinase II. Thus, our present results demonstrate that p53 binds to the beta-subunit of casein kinase II via C-terminal sequences of the p53 polypeptide chain. Binding of p53 to the regulatory subunit of casein kinase II might be implicated in the regulation of both proteins, p53 and casein kinase II.