Transcriptional regulation of the c-fos proto-oncogene requires the serum response element (SRE) which is complexed by a multi-protein assembly observed both in vitro and in vivo. Two protein factors, p67SRF and p62TCF (previously called p62), are required to interact with the SRE for efficient induction of c-fos by serum. By quantitative band shift electrophoresis we measure at least a 50-fold increase in SRE affinity for p67SRF/p62TCF over p67SRF alone. Stoichiometrically we determine that the ternary complex with p62TCF involves p67SRF in dimeric form. We demonstrate that p67SRF is a glycosylated nuclear transcription factor carrying terminal N-acetylglucosamine (GlcNAc) as a post-translational modification. A proteolytic limit digestion product, approximately 13 kd in size, was generated from the p67SRF-SRE complex. This p67SRF-core domain binds SRE, can dimerize with p67SRF and is still able to form a ternary complex with p62TCF. Therefore, three functional activities can be ascribed to this small p67SRF-core domain: specific DNA binding, dimerization and interaction with p62TCF. We demonstrate that these functions map within the p67SRF core fragment containing the region between amino acids 93 and 222.