A process for determining drug resistance of cells of interest, said process comprising the steps of: providing: (i) a previously obtained clinical or biological sample comprising cells; (ii) at least one xanthene compound that is transportable across a cell membrane by membrane transporters MDR1, MRP and BCRP; and (iii) one or more membrane transporter inhibitors; contacting cells from a first portion of said clinical or biological sample with said at least one xanthene compound; contacting cells from a second portion of said clinical or biological sample with said at least one xanthene compound and said one or more inhibitors; measuring fluorescent signals generated in step and step; and comparing the fluorescent signals measured in step to determine elevated activities of said MDR1, MRP and BCRP, or a combination thereof, thereby determining the drug resistance of said cells of interest, wherein said at least one xanthene compound has the structure wherein A and B are independently hydrogen, alkyl, cycloalkyl, or aryl; or A and B taken in combination are cycloalkyl or aryl; R 3, R 4, j, k, m, n and V are independently hydrogen, halogen, carboxy, alkoxy, aryloxy, thiol, alkylthiol, arylthiol, azido, nitro, nitroso, cyano, amino, hydroxy, phosphonyl, sulfonyl, carbonyl, boronic acid, aryl, or heteroaryl; or alkyl, or alkoxy that is itself optionally substituted one or more times by halogen, amino, hydroxy, phosphonyl, sulfonyl, carbonyl, boronic acid, aryl or heteroaryl; each R 5 is independently hydrogen, alkyl having 1-9 carbons, acetoxymethyl, or a biologically compatible salt; wherein X is independently selected from OR 12, NR 12 R 13 and Z is independently selected from O and NR 16 R 17, where each R 12, R 13, R 16 and R 17 is independently H, an alkyl having 1-12 carbons, carboxyalkyl, substituted or non-substituted amino alkyl or alkylsulfonate; and T and U are independently hydrogen, alkyl having 1-12 carbons, alkoxy having 1-12 carbons, aryloxy, amino, halogen, cyano, carboxy, acetoxymethylcarbonyl, carboxyalkyl, carbonyl, sulfonyl, phosphonyl, boronic acid, aryl or heteroaryl; provided that at least one of T and U is not hydrogen. providing: (i) a previously obtained clinical or biological sample comprising cells; (ii) at least one xanthene compound that is transportable across a cell membrane by membrane transporters MDR1, MRP and BCRP; and (iii) one or more membrane transporter inhibitors; (i) a previously obtained clinical or biological sample comprising cells; (ii) at least one xanthene compound that is transportable across a cell membrane by membrane transporters MDR1, MRP and BCRP; and (iii) one or more membrane transporter inhibitors; contacting cells from a first portion of said clinical or biological sample with said at least one xanthene compound; contacting cells from a second portion of said clinical or biological sample with said at least one xanthene compound and said one or more inhibitors; measuring fluorescent signals generated in step and step; and comparing the fluorescent signals measured in step to determine elevated activities of said MDR1, MRP and BCRP, or a combination thereof, thereby determining the drug resistance of said cells of interest, A and B are independently hydrogen, alkyl, cycloalkyl, or aryl; or A and B taken in combination are cycloalkyl or aryl; R 3, R 4, j, k, m, n and V are independently hydrogen, halogen, carboxy, alkoxy, aryloxy, thiol, alkylthiol, arylthiol, azido, nitro, nitroso, cyano, amino, hydroxy, phosphonyl, sulfonyl, carbonyl, boronic acid, aryl, or heteroaryl; or alkyl, or alkoxy that is itself optionally substituted one or more times by halogen, amino, hydroxy, phosphonyl, sulfonyl, carbonyl, boronic acid, aryl or heteroaryl; each R 5 is independently hydrogen, alkyl having 1-9 carbons, acetoxymethyl, or a biologically compatible salt; wherein X is independently selected from OR 12, NR 12 R 13 and Z is independently selected from O and NR 16 R 17, where each R 12, R 13, R 16 and R 17 is independently H, an alkyl having 1-12 carbons, carboxyalkyl, substituted or non-substituted amino alkyl or alkylsulfonate; and T and U are independently hydrogen, alkyl having 1-12 carbons, alkoxy having 1-12 carbons, aryloxy, amino, halogen, cyano, carboxy, acetoxymethylcarbonyl, carboxyalkyl, carbonyl, sulfonyl, phosphonyl, boronic acid, aryl or heteroaryl; provided that at least one of T and U is not hydrogen.