We designed a hammerhead ribozyme which site-specifically cleaved the GUC sequence in codon 179 of MDR1 mRNA. The cleavage site was 6 amino acids upstream from the drug binding site and was considered sufficiently close to the essential locus for P-glycoprotein function. The ribozyme cleaved the MDR1 mRNA under physiological conditions in vitro. The cleavage was dependent on ribozyme concentration and on incubation time. Mg2+ ion was essential for the cleavage. These results show that a potentially useful tool is at hand which may inactivate MDR1 mRNA and revert the multidrug resistance phenotype.