Abstract: ABC multidrug transporters are key players in cancer multidrug resistance and in general xenobiotic elimination, thus their functional assays provide important tools for research and diagnostic applications. It has been found that in cells expressing functional ABCG2, ABCB1, or ABCC1 transporters, cellular PG fluorescence is strongly reduced. The invention relates to methods and uses of fluorescein derivative ester compounds of formula Ia which are analogs of PG for assessing ABC transporter activity of ABC multidrug transporters. The present accumulation assay is a novel tool for the parallel determination of the function of the multidrug transporters, in particular ABCG2, ABCB1, and ABCC1. The assay is applicable for diagnostic purposes and also allows the selection, separation and culturing of selected cell populations expressing such transporters.