The immunofluorescente technique detecting antibody-coated bacteria for diagnosis of site of UTI has been applied to 52 urine samples coming from : 11 patients with clinical diagnosis of pylonephritis, 21 infections of the lower urinary tract, 8 asymptomatic bacteriurias and 2 samples of urine obtained directly by pyelocentesis. The technique confirmed to be specific and sensible: strongly fluorescent germs were observed only in samples coming from pyelonephritis or urine from pyelocentesis. Repeatedly, from single samples were isolated more bacterial strains; only one of them was present in significant quantity. The some observation has been made also in 5 patients clinically affected with P.N., where 2 strains were isolated from each sample. In only one sample both strains were fluorescent, white in the remaining 4 samples the strain present in insignificative quantity was fluorescent. These observations, rather than a double localization (pyelo-renal and lower urinary tract) with different aetiologic agents, were interpreted to be the outcome of repeated antibiotic treatments. It is possible that they could have reduced the fluorescent "bacterial load" of the pyelo-renal region under significant number, simultaneously facilitating the emergence of superinfecting germs at the level of the lower urinary tract. We discuss the therapeutic involvements for a proper antibiotic treatment and the usefulness of flanking the test at the quantitative urine culture.