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It is widely recognized that fluorescent labeling of cells is an effective means to determine total cell numbers or how many viable cells exist in a sample. Flow cytometry combined with fluorescent staining is a powerful tool to analyze heterogeneous cell populations. Fluorescein diacetate (FDA) and its derivatives are non-fluorescent molecules that diffuse into cells and are hydrolyzed by intracellular non-specific esterases to give fluorescent products. The fluorescent products can be accumulated only in those cells that have intact cell membranes; therefore, dead cells with leaky membranes are not stained. The precise kinetics of membrane transport and intracellular hydrolysis of FDA and its analogs (such as CDCFDA) are related to cellular functions, thus FDA labeling can be used for monitoring cells by flow cytometry or fluorescence microscopy. The fluorescence intensity of labeled cells by FDA dyes varies considerably among cell lines and strains, probably because of differences in intracellular esterase activity. 5-(and-6)-Carboxy-2',7'-dichlorofluorescein diacetate, succinimidyl ester, is an aminereactive FDA derivative that can be used to prepare a variety of FDA conjugates.