Viral and cellular oncogene products sometimes activate protein kinases, are protein kinases themselves, or share phosphorylation sequence motifs for different protein kinases. We have recently shown that a protein kinase activity is tightly associated with immunopurified p53. We have now expressed p53 in a baculovirus expression system and characterized this protein kinase activity in more detail. We found that casein could compete with p53 in the kinase reaction. Heparin efficiently inhibited the p53 associated protein kinase whereas the polyamine spermidine stimulated enzymatic activity. A synthetic peptide which was shown to be specifically phosphorylated by casein kinase II blocked the in vitro phosphorylation of p53, whereas a synthetic peptide with a potential phosphorylation site on human p53 at ser 315 was ineffective in blocking the phosphorylation of p53. GTP as well as ATP can be used as a phosphate donor in the in vitro kinase reaction. An antibody directed against casein kinase II coprecipitated p53 from insect cells as well as from mammalian cells. These data strongly indicate that casein kinase II is associated with immunopurified p53 and contributes to the phosphorylation of p53. A mutant p53 with a ser 389 to ala exchange was not phosphorylated in vitro by the p53 associated protein kinase.