The Chinese city of Qingdao is testing its entire population of nine million people for Covid-19 over a period of five days.
The mass testing comes after the discovery of a dozen cases linked to a hospital treating coronavirus patients arriving from abroad.
In May, China tested the entire city of Wuhan - home to 11 million people and the epicentre of the global pandemic.
The country has largely brought the virus under control.
That is in stark contrast to other parts of the world, where there are still high case numbers and lockdown restrictions of varying severity.
In a statement posted to Chinese social media site Weibo, Qingdao's Municipal Health Commission said six new cases and six asymptomatic cases had been discovered.
All the cases were linked to the same hospital, said the state-run Global Times.
The Chinese authorities now have a strategy of mass testing even when a new coronavirus cluster appears to be relatively minor, correspondents say.
How will the testing work?
The National Health Commission said on Monday that "the whole city will be tested within five days".
Some 114,862 people - including medical staff and newly hospitalised patients in the city's hospitals - had already tested negative, Qingdao's health commission said.
Videos circulating online showed residents lining up late on Sunday to get tested, said the Global Times, adding that some of these test points were open from 07:00 to 23:00.
The new cases come a week after China's Golden Week holiday - which saw millions travel across the country.
A Global Times report quoting the Qingdao Municipal Bureau of Culture and Tourism said the coastal city had received 4.47 million passenger trips over this period.
The nearby city of Jinan, in the same province as Qingdao, called for anyone who had visited the city since 23 September to get tested, according to a report by The Paper.
Earlier last month, Qingdao announced that two port workers who handled imported seafood had tested positive. They were not known to have infected anyone else.
Daily coronavirus infections have fallen drastically in China, and for the most part the country appears to have recovered from the worst.
China currently has 91,305 virus cases and the death toll stands at 4,746, according to WHO data.
How did they test Wuhan?
China said 11 million people in the city had been tested in 10 days earlier this year.
The BBC's Reality Check estimated the figure was closer to 9 million, but still a significant number of people.
Hundreds of testing centres were opened, with thousands of staff involved.
They also mobilised teams to test disabled people, the elderly and those vulnerable in their own homes.
One of the ways they sped up the process was to use batch- or pool-testing.
In this process, a batch of between five and 10 samples was tested. Only if a sample tested positive would the team go back to carry out individual tests to see who was positive within the batch.