The ability of a bacterial strain to adhere to the vaginal mucosa and the mucosa of the urinary conduits seems to be important in the pathogenesis of urinary infection. Bacterial adherence to vaginal epithelial cells was measured in vitro for 37 strains of Escherichia coli and 18 strains of Proteus mirabilis isolated from the anus, bladder and renal pelvis. No correlation could be demonstrated between the adhesive properties of a bacterial strain and its clinical pathogenicity, suggesting that host factors determining bacterial adherence are more important than bacterial adhesive factors in determining susceptibility to urinary infections.