Proto-oncogene fos encodes a nuclear protein that appears to be involved in the transcriptional regulation of some genes. fos protein (p55fos) is associated with other nuclear proteins in complexes that bind to regulatory regions containing TPA-responsive promoter elements (TREs). The enhancer-binding activator protein 1 (AP-1), which is the specific binding factor of a TRE, is the product of the nuclear proto-oncogene jun. We show that one of the fos-associated proteins, p39, is immunologically and structurally related to nuclear factor AP-1. The sequence TGACTCA, which is the consensus minimal binding site of AP-1, is also recognized by gel-purified p39. In HeLa cell extracts p55fos and p39jun are associated in a nucleoprotein complex that interacts with a TRE. Cooperation between nuclear oncoproteins fos and jun is required for full activation of transcription from a TPA-responsive promoter element in transfected mammalian cells.