The MDR1 gene product P-glycoprotein (Pgp) plays a key role in multidrug resistance of cancer cells. Pgp is an ATPdriven efflux pump that extrudes a variety of dissimilar hydrophobic cytotoxic compounds. P-glycoprotein overexpression results in multidrug resistance (MDR) of tumor cell lines in vitro as well as in cancer patients. To selectively target and eliminate MDR tumor cells, we have isolated a monoclonal antibody that specifically reacts with the first extracellular loop of the human Pgp. We have cloned the variable domain genes of this antibody and assembled a functional single-chain Fv fragment capable of specifically targeting various Pgpexpressing MDR carcinoma cells lines. Targeting and specific elimination of Pgp-dependent MDR human cancer cells was achieved by constructing a single-chain immunotoxin in which the scFv fragment was fused to a truncated form of Pseudomonas exotoxin (PE38). We conclude that recombinant Fv-immunotoxins or other Fv-based molecules armed with potent cytotoxins represent an effective tool in targeted cancer therapy aimed at specific elimination of MDR tumor cell sub-populations. Recombinant antibody fragments targeting MDR proteins such as Pgp may be also used for intracellular expression and consequent phenotypic knockout of MDR.