P fimbriae are the major single virulence factor of uropathogenic Escherichia coli strains. Recent analyses have shown that P fimbriae possess two distinct binding specificities mediated by different fimbrial subunits. P fimbriae bind to Gal alpha (1-4)Gal-containing globoseries of glycolipids of epithelial cells; this binding is mediated by the lectin-like minor protein G of the filament. In vitro mapping of the human urinary tract for binding sites of P fimbriae has revealed that they bind in a Gal alpha (1-4)Gal-inhibitable manner to epithelia of kidney and bladder. On the other hand, P fimbriae bind to immobilized fibronectin and its amino- and carboxyterminal fragments; this binding is dependent on the E and the F minor proteins of the P-fimbrial filament and seems to be based on a protein-protein interaction. The P fimbriae-fibronectin interaction has been demonstrated also on frozen sections of kidney. P fimbriae thus possess two tissue-adherence properties: one specific for epithelial glycoconjugates and the other for fibronectin of subepithelial extracellular matrices. P-fimbrial binding to epithelial glycoconjugates seems to be important in determining the host tropism and enabling the ascent of E. coli urinary tract infections. Binding to fibronectin may be important in secondary phases of the infection, e.g. after epithelial injury.