Almost 100 people in the US have now fallen ill as the result of a suspected outbreak of E. coli linked to romaine lettuce.
The US Centres for Disease Control and Prevention (CDC) says it appears that lettuce from the Yuma, Arizona region could be contaminated.
So far, 98 people have infected with the outbreak strain of the bacteria, across 22 states.
46 people have been hospitalised, while officials say cases from the last few weeks may not have been reported yet.
No deaths have been linked to the outbreak to date.
In their advice to US consumers, CDC said: "Do not eat or buy romaine lettuce unless you can confirm it is not from the Yuma, Arizona, growing region.
"Product labels often do not identify growing regions; so, do not eat or buy romaine lettuce if you do not know where it was grown."
The US Food and Drug Administration is investigating the outbreak.
It says it has identified one farm as the source of whole-head romaine lettuce that appears to have left several people ill at a facility in Alaska - but investigators do not know where in the supply chain the contamination occurred.
The FDA added: "Most of the illnesses in this outbreak are not linked to romaine lettuce from this farm. The agency is investigating dozens of other fields as potential sources of the chopped romaine lettuce."