The comparative study of more than 300 Proteus, Klebsiella, Enterobacter and Citrobacter strains isolated from patients with urological infections and parenteral infections of other localization, as well as from the feces of healthy persons has been carried out. The strains causing inflammatory processes in the urinary tract have been shown to possess no strict specificity. The ability of opportunistic enterobacteria to cause urinary tract lesions is their polydeterminant property ensured by the combination of different factors. A number of characteristics which can be considered as the markers of "nephritogenic" strains have been revealed. Thus, among Proteus mirabilis strains the largest percentage is constituted by strains fermenting sucrose and producing hemolysin. The urological pathogenicity of "nephritogenic" strains belonging to the genera Proteus, Klebsiella and Enterobacter has been found to be linked with their resistance to complement and their capacity for producing substances increasing capillary permeability. In C. freundii strains differences in O serogroups and a number of markers (the fermentation of raffinose, the formation of hemolysin and permeability factor) have been revealed. These data may be useful for the prognosis and evaluation of the course of urological infections.