The Chinese city of Qingdao is testing its entire population of nine million people for COVID-19 over a period of five days.
It comes after a dozen cases were confirmed linked to a local hospital which treats imported patients.
China had in May tested the entire city of Wuhan – a population of 11 million and the epicentre of the global outbreak.
The country has largely brought the virus under control.
That’s 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’s Health Commission said six new cases and six asymptomatic cases had been discovered.
All the cases were linked to the same hospital, said the Global Times.
The Chinese authorities now have a strategy of mass testing even when a new coronavirus cluster appears to be relatively minor, correspondents said.