Security is being increased on New York's underground railway, as city chiefs say they've received information about a believable terrorist threat.
Patrols and searches are being stepped up in the US city, and Police chief Ray Kelly has asked travellers to not use bags, pushchairs or briefcases.
Mayor Michael Bloomberg said the threat related to the next few days, and was the most specific yet.
However, government officials say they don't think there is much danger.
Mayor Bloomberg asked New Yorkers to keep their eyes out for any danger.
He said he was still confident enough to use the subway, which is used by 4.5m people every day.
The city's threat level is still orange, the second highest level of danger, and the level it has been at since the 11 September attacks on the World Trade Centre.