An in vitro system was used for assay of the attachment of Escherichia coli to viable uroepithelial cells harvested from the urine of premenopausal women. A 15% level of variation from the mean attachment value was noted within given cell samples. Uroepithelial target cells with large numbers of adherent bacteria were found. The receptivity of uroepithelial cells to the attachment of certain strains of E coli varied over the menstrual cycle and during pregnancy. This finding may suggest that adult women are more susceptible to urinary tract infection at particular times during their reproductive period. Double-staining techniques were used for the detection of bacterial attachment to mucopolysaccharide-coated and non-mucopolysaccharide-coated uroepithelial cells. The level of attachment of one strain of E coli was higher when cells were mucus coated, while the attachment of a second strain was unaffected by the presence of a mucus coat. The results suggest the existence of at least two different mechanisms whereby E coli attaches to uroepithelial cells.