A hybridoma (termed "UIC2 hybridoma", ATCC Accession No. HB11027) producing monoclonal antibodies (termed "UIC2 mAb") directed against an extracellular domain of a cell surface P-glycoprotein antigen associated with multidrug resistance in primate cells was produced by fusing a human myeloma cell with a spleen cell derived from a BALB/c mouse immunized with syngeneic 3T3 fibroblasts previously transfected with the isolated human mdr1 cDNA. UIC2 mAb, thus produced, as well as fragments and recombinant derivatives thereof, may be used to detect and isolate multidrug resistant primate cells and human mdr1 gene products, and to reverse multidrug resistance in primate cells, including cells of multidrug resistant human tumors.