A simple fluorometric assay requiring only a single sample of cells to determine the number of cells, from the DNA linked to the fluorochrome 4',6-diamidino-2-phenylindol (DAPI), and the uptake of ciprofloxacin, a natural fluorescent quinolone is described. Mouse peritoneal macrophages were found to concentrate ciprofloxacin up to 12.7 (+/- 1.5)-fold. Combined fluorometry provides a precise, sensitive method for determining the intracellular concentration of fluoroquinolones, as well as that of naturally fluorescent or fluorochrome-linked drugs.