Two strains of Escherichia coli that formed on unusual kind of mannose-resistant and eluting haemagglutinin (MREHA) reacting with the red blood cells of rat and mouse, when cultured at 37 degrees C but not at 18 degrees C, were examined by electron microscopy. Production of this rare rodent-positive MREHA was correlated with the presence of fine fibrillae of estimated diameter 2.5 nm that were demonstrated by negative staining and immuno-gold labelling with MREHA-specific anti-serum. These two strains belonged to serotypes 078:H- and 078:H33; thus, it would be useful to know whether enteroadherent-aggregative strains of E. coli of these and other serotypes also possess this unusual MREHA.