We have cloned the human alpha 1-antitrypsin (alpha 1-AT) gene and identified the promoter and the transcription initiation point. The cloned gene, following transfection, is expressed in a cell-specific manner, being transcribed in a human hepatoma cell line (Hep3B) but not in HeLa cells. We show that the 5' flanking region of the alpha 1-AT gene contains DNA sequences sufficient for efficient transcription in Hep3B but not in HeLa cells. This DNA sequence also activates, in a cell-specific manner, heterologous promoters such as that of SV40; however, the effect is only obtained in one orientation, suggesting that this cis-acting cell-specific element does not share all the features generally associated with enhancers. By cotransfection-competition experiments we also show the existence of a limiting trans-acting factor, essential for the expression of the alpha 1-AT gene in Hep3B cells.