The enhancer-binding protein AP-1 has been purified to greater than 95% homogeneity from HeLa cells by sequence-specific DNA affinity chromatography and identified as a 47 kd polypeptide. Purified AP-1 activates transcription in vitro of the wild-type human metallothionein IIA (hMT IIA) gene but not mutant hMT IIA promoters lacking AP-1 recognition sites. DNAase I protection analysis indicates that genetically defined enhancer elements in hMT IIA, SV40, and the human collagenase gene contain high-affinity AP-1-binding sites, each with a conserved recognition motif, TGACTCA. These three genes are transcriptionally induced by treatment of cells with the tumor promoter TPA. Here we demonstrate that multiple synthetic copies of the consensus AP-1-binding site can act as TPA-inducible enhancers in various plasmid constructs after transfection into HeLa cells. These findings suggest that AP-1 is a transcription factor that functions by interacting with a specific enhancer element, and that its activities may be modulated by treatment of cells with TPA, known to stimulate protein kinase C.