Flow cytometry has rapidly evolved from a technique for detecting and sorting cells on the basis of their DNA content or immunological markers into a useful tool for detection and quantitation of intracellular drug fluorescence (2 ,25 ,30 ,37 ). Recent studies have shown that intracellular content of fluorescent drugs can be rapidly quantitated on a cell-to-cell basis by this sophisticated analytical method. Thus, one can analyze intracellular drug transport (influx, efflux), retention, and/or binding, and correlate these parameters with effects on cellular metabolism and proliferation (21 ,22 ).
