To investigate multidrug-resistance gene (MDR1) promoter efficacy and drug inducibility in cells with different multidrug-resistance phenotypes, multidrug-resistant HCT15 and drug-sensitive KM12 human colon carcinoma cell lines were transfected with constructs incorporating the chloramphenicol acetyltransferase (CAT) reporter gene, driven by wild-type and point-mutated MDR1 promoter regions. The basal CAT expression level in HCT15 cells was markedly elevated compared to KM12 cells. CAT induction by vincristine was dose-dependent over a broad concentration range (40-500 ng/ml) in both lines. The induction levels were related to the cells' MDR phenotype, with the multidrug-resistant HCT15 cells showing the greater effect. In both cell types, basal and drug-induced CAT expression were significantly enhanced by the point-mutated promoter regions. The findings support the possible exploitation of the MDR1 promoter for construction of drug-inducible and MDR-cell-targeted expression vectors for use in gene therapy.