In this study, the efficiency of inducible promoters to switch on gene expression in the presence of inducer or to switch it off in its absence was evaluated in tobacco cell suspensions transformed with the gus gene coding sequence. Either plant (pats1A, pSalT, pIn2-2) or microbial (pMre, pTet) inducible promoters were used to drive gus expression. The inducers were light, abscisic acid, 2-CBSU, CuSO4, tetracycline, respectively. For each construct (inducible promoter-gus coding sequence), the optimal induction conditions were determined (inducer concentration, induction time, and age of cells in culture cycle before induction). The efficiency of the inducible promoter was then evaluated under optimal induction conditions. GUS-expression levels obtained under noninducing and inducing conditons were systematically compared. Thirty or forty percent of the clones transformed with the pSalT-gus or pTet-gus construct, respectively, showed high induction rates (>1000) and GUS activities of the same order as those obtained with a constitutive system. However, basal GUS levels were always high for the pTet-gus cell lines. Seventy or eighty-five percent of the cell lines transformed with the pMre-gus or pIn2-2-gus construct, respectively, had induction rates of 1.5 to 1000. The pats1A-gus construct gave very low induction rates-55% of cell lines had induction rates less than 1.5. Only the pSalt-gus construct gave both the highest induction rates and basal GUS-levels equivalent to the endogenous GUS background.