Utilization of chemotherapy for the treatment of tumors is mainly limited by its hematological toxicity. Because of the low-level expression of drug resistance genes, transduction of hematopoietic progenitors with multidrug resistance 1 (MDR1) or multidrug resistance-associated protein (MRP) genes should provide protection from chemotherapeutic agent toxicity. Successful transfer of drug resistance genes into hematopoietic cells may allow the administration of higher doses of chemotherapy and, thus, increase regression of chemosensitive tumors. The interest in the use of MRP as an alternative to MDR1 for bone marrow protection lies in its different modulation. This would allow, in the same patient, the use of MDR1 reversal agents to decrease MDR1 tumor resistance without reversing bone marrow (BM) protection of the MRP-transduced hematopoietic cells, since MRP expression is not reversed by these agents. We have constructed MRP-containing retroviral vectors using the phosphoglycerate kinase promoter and generated ecotropic producer cells. Lethally irradiated mice were engrafted with BM cells transduced by coculture with MRP producer cells. Evidence of long-term (9 months) gene transfer was provided by PCR of peripheral blood from MRP-transduced mice. Southern blot analysis confirmed the integrity of the provirus in the MRP-transduced mice. Long-term MRP expression (>5 months) was detected by RT-PCR and fluorescence-activated cell sorting of blood from living mice. High-level expression of MRP in murine hematopoietic cells reduces doxorubicin-induced leukopenia and mortality. Furthermore, we show in vivo selection of MRP-transduced cells following doxorubicin administration, with better and more significant chemoprotection after the second chemotherapy cycle. These data indicate that MRP retroviral gene transfer may be useful for chemoprotection and selection.