Accumulation of norfloxacin by Escherichia coli was studied with a range of published procedures that used either radioactively-labelled norfloxacin (14C and 3H) or the natural fluorescence of the quinolone for detection. All methods except bioassay generated comparable data. A method involving the detection of fluorescence was found to be the method of choice. This method was used to study the accumulation kinetics of ciprofloxacin, lomefloxacin, fleroxacin, norfloxacin, and enoxacin by several species of Gram-negative bacteria, and a Staphylococcus aureus strain. Saturation and efflux kinetics were also studied. There was no saturation at a concentration of norfloxacin less than 50 mg/L. Norfloxacin efflux was minimal during the uptake assay as the samples were withdrawn into ice-cold buffer; however, when the cells were sampled into buffer at 37 degrees C, up to 50% of cell-associated quinolone effluxed within 5 min.