Single-dose gentamicin (80--120 mg; Refobacin) can be used successfully to treat significant bacteriuria in women, while at the same time differentiating between upper and lower urinary tract infection. Localization was achieved by demonstrating antibody-carrying bacteria in urine by immunofluorescence. In all 31 women with lower urinary-tract infection so treated urinary sample continued to be bacteria-free when retested one to for weeks later. On the other hand, in 22 of 25 women with renal bacteriuria the original bacteria were repeatedly isolated after treatment. Using single-dose gentamicin and subsequently re-testing urine samples, renal infections in four women were correctly diagnosed in this manner, although the initial immunofluorescence test was negative.