The serum response element (SRE) is essential for serum and growth factor stimulation of the c-fos gene. We have examined the nuclear proteins, obtained from tissues with elevated expression of the c-fos gene (proliferating rat liver and hepatocarcinoma), that bind to the SRE sequence. A synthetic oligonucleotide containing the SRE sequence from the mouse c-fos gene promoter (-299 to -322) was radioactively labeled, used as a probe for the mobility shift assay and Southwestern (DNA-protein) blotting, and also used for sequence-specific affinity chromatography. We have identified a group of nuclear proteins of molecular sizes 36, 45, 62, 67, 72, and 112 kDa capable of interacting with the SRE sequence. The 36-, 67-, and 112-kDa proteins have DNA-binding properties, but the presence of the others in the SRE-protein complex could be the result of protein-protein interaction. All of these protein factors were present in nuclei obtained from intact and proliferating rat liver as well as from 5123tc Morris hepatoma. The DNA-binding activity (on Southwestern blots) of the 67- and 112-kDa proteins was not affected by alkaline phosphatase treatment, but the ability of the dephosphorylated nuclear proteins to form the complex with the SRE sequence under gel shift assay conditions was severely impaired. The same alkaline phosphatase treatment completely abolished the DNA-binding properties of the c-fos cyclic AMP-responsive element-specific proteins. Therefore, transcriptional activation of the c-fos gene at the SRE must require the presence of a multiprotein complex the formation of which is governed by phosphorylation. The binding of the 67- and 62-kDa proteins to the c-fos SRE has been previously reported; however, the 36-. 45-, 72-, and 112-kDa proteins are novel factors involved in the multifaceted regulation of c-fos gene expression in vivo.