Moldovan police have seized 1.8kg of uranium-238 in the capital, Chisinau, interior ministry officials say.
Seven people were arrested, including former interior ministry officials, the ministry said.
The smugglers had reportedly been trying to sell the material on the European black market for 9m euros (£7.4m).
The source and intended destination of the uranium were not clear, investigator Oleg Putintica said.
It was found in a garage in Chisinau, where it was under guard and in a special container, said interior ministry spokesman Chiril Motpan.
He said there were seven suspects, some of whom had previous convictions for possessing radioactive materials in Moldova, Russia and Romania.

Seven people were arrested, including former interior ministry officials, the ministry said.
The smugglers had reportedly been trying to sell the material on the European black market for 9m euros (£7.4m).
The source and intended destination of the uranium were not clear, investigator Oleg Putintica said.
It was found in a garage in Chisinau, where it was under guard and in a special container, said interior ministry spokesman Chiril Motpan.
He said there were seven suspects, some of whom had previous convictions for possessing radioactive materials in Moldova, Russia and Romania.