Upon DNA damage, the amino terminus of p53 is phosphorylated at a number of serine residues including S20, a site that is particularly important in regulating stability and function of the protein. Because no known kinase has been identified that can modify this site, HeLa nuclear extracts were fractionated and S20 phosphorylation was followed. We discovered that a S20 kinase activity copurifies with the human homolog of the Schizosaccharomyces pombe checkpoint kinase, Chk1 (hCHK1). We confirmed that recombinant hCHK1, but not a kinase-defective version of hCHK1, can phosphorylate p53 in vitro at S20. Additional inducible amino- and carboxy-terminal sites in p53 are also phosphorylated by hCHK1, indicating that this is an unusually versatile protein kinase. It is interesting that hCHK1 strongly prefers tetrameric to monomeric p53 in vitro, consistent with our observation that phosphorylation of amino-terminal sites in vivo requires that p53 be oligomeric. Regulation of the levels and activity of hCHK1 in transfected cells is directly correlated with the levels of p53; expression of either a kinase-defective hCHK1 or antisense hCHK1 leads to reduced levels of cotransfected p53, whereas overexpression of wild-type hCHK1 or the kinase domain of hCHK1 results in increased levels of expressed p53 protein. The human homolog of the second S. pombe checkpoint kinase, Cds1 (CHK2/hCds1), phosphorylates tetrameric p53 but not monomeric p53 in vitro at sites similar to those phosphorylated by hCHK1 kinase, suggesting that both checkpoint kinases can play roles in regulating p53 after DNA damage.