Escherichia coli isolated from faeces of 54 healthy volunteers who visited Tunisia for eight days were examined. These volunteers participated in a randomized double-blind placebo-controlled study to establish whether ciprofloxacin could prevent travellers' diarrhoea. Escherichia coli strains isolated before travel, during episodes of travellers' diarrhoea, immediately after return and five weeks after return were serotyped and tested for the presence of virulence genes indicating diarrheogenic properties by hybridization with a set of four non-radioactively labelled DNA probes. Subjects receiving ciprofloxacin prophylactically to prevent travellers' diarrhoea were asymptomatic and no Escherichia coli could be cultured shortly after return home. Sixty-four percent of subjects (18/28) who did not receive antibiotic prophylaxis suffered from travellers' diarrhoea. Hybridization tests detected 8 enterotoxigenic Escherichia coli strains producing heat stable toxin, 13 enterotoxigenic strains producing heat labile toxin and 10 strains which produced both heat labile and stable toxin. Of the 31 probe positive strains, 29 (94%) were cultured from 11 volunteers with travellers' diarrhoea. A bacterial cause was thus determined in 61% of the volunteers who experienced travellers' diarrhoea.