Formalin-fixed human ileal mucosa and formalin-fixed or untreated (native) human urinary bladder mucosa were used to test the adherence ability of Escherichia coli enterotoxigenic (ETEC) or uropathogenic (UPEC) for humans. When grown on colonization factor antigen (CFA) agar for 3 h at 37 degrees C, ETEC with CFA/I or CFA/II pili had typical peritrichous flagella and adhered strongly to human ileal lymphoid follicle and villus epithelium. In contrast, E. coli cells with CFA/I or CFA/II pili and possessing very weak or no motility displayed low levels of adherence to the epithelium. UPEC, which possessed type 1 pili and rarely had flagella, strongly adhered to human urinary bladder mucosa but not to human ileal epithelium. Type 1 pili-possessing E. coli isolated from human feces behaved as did UPEC. Moreover, M cells (microfolds) present in human ileal lymphoid follicle epithelium provided adherence sites for type 1 pili but not for CFA/I or CFA/II pili. These data demonstrate the importance of bacterial motility in efficient in vitro adherence to human ileal epithelia, in contrast to human urinary epithelia, and the adhesin specificity of bacterial adherence to M cell microfolds.