The MDR1 gene product P-glycoprotein (P-gp) extrudes several anticancer drugs including taxol and fluorescent dyes such as rhodamine (Rh123). Modulation of the level of P-gp expression has the potential of overcoming multidrug resistance. One possible approach is the retroviral transfer of the human MDR1 gene into murine and human bone marrow (BM) progenitor cells. The rationale for this approach is increased chemoprotection, which allows chemotherapy of a greater level of intensity to be delivered. In this study, flow cytometric measurement of Rh123 extrusion was used to test P-gp function in human and mouse haemopoietic progenitor cells, which had been transduced with a virus containing the human MDR1 transcription unit. Human CD34+ selected cells were analysed immediately following transduction. In two successive experiments MDR1 cDNA transduction resulted in a 7% and 11% increase of P-gp expressing Rh123 dull cells. To monitor transduction efficiency over time as well as the possibility of in vivo selection of drug-resistant BM cells in mice treated with increasing numbers of taxol cycles, the assay was also successfully applied to peripheral blood lymphocytes of mice transplanted with MDR1 transduced BM cells, demonstrating increased Rh123 efflux in transduced cells. Analysis of another fluorescence assay using fluorescein di-beta galactopyranoside as a substrate for beta-galactosidase in cells transduced with a MDR1: beta-gal activity. We conclude that the Rh123 efflux assay is a sensitive method to monitor P-gp function in MDR1 cDNA transduced cells, and may be used to enrich transduced cells via flow cytometric cell sorting for Rh123 dull cells.