Heat-labile enterotoxin produced by enterotoxigenic E. coli affected the structure of epitherial cell of rabbit intestine. The toxin bound to the microbilli of the cell and caused "puffing" of the microvillous membrane when it was examined under phasecontrast microscopy. The same change occured by the isolated brush border membrane if it was reacted with the enterotoxin in the presence of ATP. Electronmicroscopic observation disclosed that the bundle fibrous structure of the brush border was deformed to random structure and that some parts of the fibrous bundle detached from microvillous membrane. Since the enterotoxin bound to microvilli, it was suggested that cytoskelton including actin and myosin system of the cell could be affected through cAMP of which synthesis was activated at the microvilli membrane by the enterotoxin.