Two hundred and ninety-nine different strains of Escherichia coli, isolated from 172 patients with diarrhoea and 113 healthy subjects, were examined for enterotoxin, cytotoxin and haemolysin (Hly) production and for mannose-resistant haemagglutination (MRHA) and invasive properties. Three strains proved enterotoxigenic, none enteroinvasive; cytotoxin and Hly production was shown in 25 strains from patients and in 3 from controls. Ten strains produced the cytotoxic necrotizing factor (CNF), 6 released other factors which kill cell cultures. Hly production was shown in 21 strains, 9 of which were also positive for CNF. MRHA was detected in 26% of strains from diarrhoca compared with 14% of strains from healthy people. A strong association between toxin production and MRHA was demonstrated. Serotyping results showed that the strains exhibiting virulence traits mostly belonged to serogroups commonly involved in extra-intestinal infections. The possible role of strains of E. coli showing one or more virulence factors as opportunistic pathogens in diarrhoeal diseases is discussed.