Both, the activity as well as the expression of protein kinase CK2 is enhanced in various cancer types and in established tumour cell lines. This phenomenon is not due to an increase in the CK2 message but rather to posttranscriptional and posttranslational mechanisms. In order to get an insight into these posttranslational modifications we analyzed CK2 in prostate cancer cell lines, which differ by their hormone-sensitivity. We found that the CK2 activity is significantly higher in hormone-refractory than in hormone-sensitive cells although the amount of the catalytic alpha- and alpha'- subunits is comparable. In contrast, we detected seemingly lower amounts of the regulatory beta-subunit in the hormone-refractory cell lines, which later turned out to be an immunologically defined subclass. This subclass is realized by a phosphate group, which is attached to serine 209. The phosphorylation occurs in vivo during mitosis and is executed by the p34(cdc2)/cyclin B kinase. As this phosphorylation enhances the CK2 activity this change might well account for the higher activity of CK2 in prostate cancer cells.