During six days in November, 1987, 611 pupils (age range 7-19 years) and 39 adults (23-57) at a school complex in southern Finland had diarrhoea due to Escherichia coli O111:B4. Diarrhoea developed in 137 other household members during the two weeks after the school outbreak. The source of the organism remains unknown. The outbreak strains, when incubated at 22 degrees C or exposed to ampicillin, lost the lipopolysaccharide O antigen and began to react with antisera against Salmonella typhi Vi antigen. The Vi antigen-like reactivity increased the adherence of the organisms to Hep-2 cells. These results indicate that E coli O111:B4, and possibly other enteropathogenic E coli strains, should be considered in the diagnosis of all diarrhoea cases and not only in infantile diarrhoea. Expression of Vi antigen in E coli may play a part in virulence by enhancing adherence to the intestinal epithelium.