The six E. coli isolates assayed for ciprofloxacin accumulation showed varying accumu- lation levels (Figure 1b). Elution at 20 min showed that these isolates accumulated between 6.5 and 20 ng ciprofloxacin/mg cells, with an average of 11.4 ng/mg cells. After CCCP addition, these values increased to 30–82 ng ciprofloxacin/mg cells, averaging 46.7 ng/mg. The difference in accumulation ranged from 10.0 to 75.5 ng/mg, with the latter being demonstrated by isolate F18. The concentrations of ciprofloxacin accumulated for E. coli isolates at 120 min were 2.5–28.5 ng ciprofloxacin/mg cells, with an average of 13.8 ng/mg. At 120 min, post-CCCP addition, the concentrations of ciprofloxacin accumulated were in the range of 37–89 ng ciprofloxacin/mg cells, with an average of 53.8 ng/mg. The difference in accumulation was calculated to range from 86.5 to 115.5 ng ciprofloxacin/mg cells. The greatest level of accumulation at this time was for isolate F18.