Small interfering RNAs (siRNAs) are powerful tools in specifically silencing gene expression. Nevertheless, their efficiency can be limited when targeting proteins with an unusually long half-life, such as P-glycoprotein (P-gp), which is involved in the multidrug resistance phenomenon. P-gp is characterized by a long half-life, which may vary depending on the cell line and, for some of them, on serum deprivation or high cell density. In the present paper, involvement of an exponential cell growth phase in the improvement of siRNA efficiency has been suggested. The doxorubicin-selected human line MCF7-R was shown to be a more adapted model than NIH-MDR-G185 cells stably transfected with human mdr1. Nonspecific effects occurring at moderate (100 nM) siRNA concentration have been shown. Two efficient siRNAs led to a very satisfactory P-gp extinction (only 20% P-gp expression remaining) with siRNA concentration as low as 20 nM.