Faecal samples from 350 farm workers on 276 dairy farms and 50 abattoir employees from seven different operations were examined for the presence of Verocytotoxin-producing Escherichia coli O157 (VTEC O157) by an O157-specific enzyme-linked fluorescent assay followed by immuno-concentration. VTEC O157 was isolated from four (1·1%) of the farm workers. A second stool sample was obtained from the positive farm workers as well as from their household contacts. VTEC O157 was isolated from the wife of one of them. The strains from the same household shared the same Verocytotoxin genes profile, phage type and pulsed-field gel electrophoresis pattern. The VTEC O157-positive subjects had neither intestinal symptoms at the moment of sampling nor a history of bloody diarrhoea or renal failure. Our study seems to confirm the hypothesis that farm residents often develop immunity to VTEC O157 infection, possibly due to recurrent exposure to less virulent strains of VTEC.