A number of transcription factors have been shown to be phosphorylated by casein kinase II (CKII). We have identified CKII phosphorylation sites in c-Myc, Max, and c-Myb which are phosphorylated in the cell. Whereas little evidence to any functional significance of the CKII sites in c-Myc has been obtained, phosphorylation of its heterodimeric partner Max alters DNA binding properties. CKII phosphorylation of Ser-2 and -11 in Max resulted in enhanced DNA binding kinetics of both Max/Max homo- and Myc/Max heterodimers without altering steady state binding. Replacing these serine by alanine residues and comparing the wild type with the mutant Max proteins in transactivation assays did not reveal any significant differences. For c-Myb mutational analysis of the CKII phosphorylation sites showed altered steady state DNA binding. Replacing Ser-11/12 by alanine residues resulted in increased DNA binding compared to wt c-Myb or Myb Asp-11/12 as demonstrated by up to 10-fold differences in the dissociation constants. In transactivation assays, the Ala mutant showed consistently an increased activity both on a synthetic and on the mim-1 promoter. A potential CKII phosphorylation site in c-Fos was not phosphorylated in vitro. Analysis with peptides demonstrated that a proline residue at position +1 relative to the acceptor serine was inhibitory.