PEA3, a member of the Ets family of transcription factors, is a nuclear phosphoprotein capable of activating transcription. Mouse PEA3 comprises 480 amino acids and bears an approximately 85-amino acid ETS domain near its carboxyl terminus. Whereas analyses of bacterially expressed PEA3 revealed that the ETS domain is required for sequence-specific DNA binding, little is known of the functional domains in the protein required for its activity in mammalian cells. To this end, we defined the location of the PEA3 functional domains in COS cells. PEA3 bears a strong activation domain near its amino terminus, which is flanked by two regions that independently negatively regulate its activity. PEA3 expressed in COS cells was incapable of binding to DNA in vitro. However, DNA binding activity could be unmasked by incubation with a PEA3-specific antibody. Analyses of the DNA binding activity of PEA3 deletion mutants revealed that two regions flanking the ETS domain independently inhibited DNA binding; deletion of both regions was required to detect DNA binding in the absence of a PEA3-specific antibody. Under these conditions, the ETS domain was sufficient for sequence-specific DNA binding. These findings suggest that the activity of PEA3 is exquisitely controlled at multiple functional levels.