Genomic clones coding for human fibroblast collagenase were isolated. By constructing and transfecting mutants with 5' and 3' deletion mutations of the 5' control region of the gene into human or murine cells, we delimited a 32-base-pair sequence at positions -73 to -42 which is required for the induction of transcription by the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate. The DNA element behaves as a 12-O-tetradecanoyl-phorbol-13-acetate-inducible enhancer: it mediates the stimulation of transcription to the heterologous herpes simplex virus thymidine kinase promoter and acts in a position- and orientation-independent manner. Differences in enhancer efficiency in different cell lines are interpreted to indicate differences in the activity of a trans-acting factor.