In this work, we have studied the activity of a tetracycline modulatable trans-activator (tTA) generated by fusing the DNA binding domain of the tetracycline repressor to the trans-activation domain of the Herpes simplex virus protein 16 (HSV VP16) (plasmid pUHD15-1Neo). In the three different cell lines studied (HTC, rat hepatoma; T47D, human breast cancer; SK-N-BE, human neuroblastoma), the expression of the luciferase gene under the control of a tetracycline operator sequence (plasmid pUHC13-3) was used as a control of the incorporation and the functionality of the trans-activator. Clones selected from these cells responded in a time and dose-dependent manner to the withdrawal of tetracycline. In all these clones, the tTA trans-activator not only modulates the activity of the luciferase gene, but also modulates the activity of a number of endogenous proteins, including C/EBP beta, the glucocorticoid receptor (GR), and SP1. In the transfected cells, the level of these transcription factors was strongly inhibited in the presence of tetracycline and was highly increased after tetracycline removal. Electrophoresis mobility shift assay (EMSA) and footprint experiments proved that the induced proteins are perfectly efficient in binding the DNA. Their transcriptional activity was also determined. In HTC/A9 cells, the level of the chloramphenicol acetyltransferase (CAT) expression driven by the promoter of the alpha 1-glycoprotein (AGP) gene was strongly enhanced at 72-84 hr following removal of tetracycline from the growth media. The accumulation of the endogenous AGP mRNA also increased at 84 hr. In the T47D/TA11 and SK-N-BE/C2.6 cells, a general activation of protein synthesis was also evidenced.