DNA tester kits are being introduced in central London's Tube stations to help prosecute people who spit at staff.
London Underground employees have complained of being spat at up to 100 times in the past year, but they can prove difficult to prove in court, says LU.
Staff have been given a pair of sterile latex gloves, a self-sealing bag and two sterile swabs to collect a sample to prevent the evidence being lost.
The project by LU and British Transport Police follows the success of a similar scheme on ScotRail in Glasgow.
John Strutton, of LU's crime and disorder unit, said: "We want to make it
clear that we will not tolerate violence of any kind against Tube staff."
"They do an excellent job in sometimes difficult circumstances, but whatever the situation this does not justify assault of any kind, including spitting."
If the scheme is successful in central London over the next few months, it will be rolled out across the rest of the Tube network.