Moldovan police say they have arrested seven people suspected of smuggling uranium from Russia to sell it on.
About 200g (7oz) of the material was seized in a raid last week, with a black market value of about $2.1m (£1.3m; €1.7m).
It was allegedly smuggled into Moldova in a special container on a train.
Those arrested had experience of handling radioactive substances, and have admitted their guilt, Moldovan police added.
A kilogram of another hazardous material, mercury, was also seized.
The US Federal Bureau of Investigation aided the police in their investigation, the Associated Press news agency reports.
In 2011, Moldovan police arrested six people trying to sell a kilogram of uranium to buyers in Africa.