A study was made of 268 cultures isolated from the urine of 263 children suffering from pyelonephritis. Of the total number of different cultures E. coli constituted 79.3 percent; the percentage of the rest varied from 5.2 to 0.4. Examination of 87 urinocultures of E. coli isolated from sick children with the specific immune response showed that the majority of bacterial signs (urease activity, capacity to produce alpha-hemolysin to utilize saccharose and raffinose, to synthesize colicine) failed to correlate with their pyelopathogenicity. The reference to individual serological groups also failed to serve as a sufficient foundation for the separation of these microbes into individual nephropathogenic or pyelopathogenic groups. In experiments with 3H-glucose labeled bacteria there was revealed a marked adhesive capacity in 94 percent of E. coli strains towards the epithelial cells of the RH strain. A positive radioactive label failed to correlate with the presence in E. coli of common pili and with the bacterial agglutinability with the sera K88, K99, and KH-III. The latter pointed to the presence of a factor of unknown nature in the nephropathogenic E. coli strains imparting adhesive properties to bacteria.