Wild-type mouse p53, expressed in Escherichia coli, was phosphorylated by highly purified casein kinase I (CKI) from rabbit muscle. The major site of phosphorylation in the p53 was identified as serine 6, which is known to be phosphorylated in vivo. Serines 4 and 9 were also phosphorylated. To determine whether CKI is likely to be a physiological p53 kinase, SV3T3 cell lysates were fractionated on a Mono Q column and assayed for p53 kinase and casein kinase activities. Four p53 kinase activities were detected, one of which co-purified with CKI activity. This p53 kinase (designated PK270) further co-purified with CKI on sucrose gradients and had a native molecular weight, like CKI, in the range of 35,000-45,000. However, PK270 was separated from the bulk of CKI activity on a phosvitin-Sepharose affinity column, and was therefore likely to be a CKI-related kinase. In support of these conclusions, phosphorylation of p53, by both CKI and PK270, was inhibited by a peptide corresponding to a consensus CKI target sequence, but not by a non-specific peptide. Moreover, phosphopeptide analyses of p53 phosphorylated by CKI or by PK270 gave similar results, indicating that these two kinases phosphorylate the same sites in p53.