expression of the mdr3 gene in prolymphocytic leukemia association with cyclosporinainduced increase in drug accumulation typical multidrug resistance in human and animal cell lines is caused by overactivity of an unidirectional transmembrane drug efflux pump encoded by the mdr genes called mdr genes in mice and humans and pgp genes in hamsters in humans two mdr genes mdr1 and mdr3 with approximately 80 nucleotide homology have been identified there is increasing evidence that overexpression of the mdr1 gene plays a role in resistance to anticancer agents in specific tumor types however currently no data are available on a possible role for mdr3 in drug resistance here we report high levels of expression of mdr3 gene sequences in leukemic cells from 6 out of 6 patients with prolymphocytic leukemia pll no mdr1 expression was detected in 5 out of 6 of these samples whereas a low level of mdr1 expression was found in a sample from one pll patient in the course of transformation to nonhodgkins lymphoma except for this patient all other pll cases studied had not received prior chemotherapy in vitro drug uptake studies showed that daunorubicin accumulation in pll cells was increased by cyclosporin a since cyclosporin a is an inhibitor of the mdr1encoded pglycoprotein drug pump these data suggest that in pll cells mdr3 also codes for a drug efflux pump our findings could partly explain the primary refractoriness of pll to chemotherapy