1. Five 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins), (e.g. atorvastatin, fluvastatin, lovastatin, pravastatin and simvastatin), were investigated for their ability to reverse P-glycoprotein (P-gp) mediated rhodamine 123 (R123) transport in a murine monocytic leukaemia cell line that over-expresses the multi-drug resistance protein 1a/b (mdr1a/1b). 2. P-gp modulation was studied by a fluorimetric assay and confocal microscopy by means of R123 efflux and uptake experiments, respectively. 3. Atorvastatin acid, methyl ester and lactone, lovastatin lactone and simvastatin lactone inhibited R123 transport in a concentration-dependent manner. Lovastatin acid, simvastatin acid, fluvastatin and pravastatin did not show a significant inhibition of the R123 transport in our cell system. Atorvastatin methyl ester and lactone showed the highest affinities for P-gp and results were comparable for both methods. 4. In conclusion, monitoring of R123 transport in living cells by confocal microscopy in addition to fluorimetric assay is a sensitive tool to study P-gp affinity in drug screening that is especially useful for early phases of drug development.