Optimal conditions for in vitro adherence of Escherichia coli to uroepithelial cells, previously shown to more efficient for strains causing acute symptomatic than that for strains causing "asymptomatic" urinary tract infections, were investigated. Uroepithelial cells from fresh morning urine of healthy individuals and E. coli bacteria from patients with various forms of urinary tract infeciton were used. Adhesion was found to vary, between individuals and epithelial cell types, with epithelial cell viability, bacterial cultivation medium and growth phase, number of bacteria added to the epithelial cells, and incubation time and temperature. Adhesion was also influenced by variations in pH and osmolarity. Optimal test conditions were obtained with post-log-phase bacterial cultures grown on nutrient broth when 10(8) bacteria were added to 10(5) epithelial cells and incubated for 60 min. Considerable variation was found between experiments done on different days, whereas the variation between duplicates was small. The method described may provide a useful tool in the study of the host-parasite relationship in urinary tract infections.