Fifty-four strains of Escherichia coli belonging to serogroup O 157 were examined for the production of Vero cytotoxins VT1 and VT2, and for other properties such as plasmid content, resistance to antimicrobial agents and colicin production. Twenty-six strains from cases of diarrhoea, haemorrhagic colitis and haemolytic uraemic syndrome in humans produced VT. By serum neutralization tests and hybridization with DNA probes for VTl or VT2, three classes were recognized which produced either VTl alone or VT2 alone or both VTl and VT2. These strains were of H type 7 or non-motile. The strains producing VT were sensitive to all the antimicrobial agents tested, and all carried at least one plasmid which had a molecular weight of c.60 × 106. Seven strains of porcine origin and 21 strains of human origin did not produce VT or hybridize with either DNA probe. None of these strains was of H type 7. Of the 21 human VT” strains, 17 were of extraintestinal origin and 18 were of H type 45. Twenty-three of the 28 VT” strains were resistant to at least one antimicrobial agent.