c-Myb is the prototype of a family of transcription factors characterised by a unique DNA binding domain. Previous analyses have concentrated on truncated versions of c-Myb as it has been very difficult to produce full length c-Myb. To overcome these difficulties we expressed full length c-Myb in HeLa cells using a recombinant vaccinia virus. Partially purified native full length c-Myb bound efficiently and specifically to DNA with a dissociation constant similar to that obtained with bacterially expressed DNA binding domains. No evidence was found for a negative effect of the leucine zipper on DNA binding. Furthermore the DNA binding domain was protease resistant in contrast to the transactivation and negative regulatory domains. Phosphorylation had no apparent effect on this differential protease sensitivity. The increased sensitivity of the C-terminal domain suggests a more open conformation, which may be relevant in the integration of signals and/or in protein-protein interactions.